You are absolutely right - a mix is good.

Band 6 asks for "a mix of simple and complex sentence forms" and Band 7 looks for "a variety of complex structures."  Some people think this means ONLY complex structures in Band 7 - NO!  It means a mix! Band 8 looks for writers who use "a wide range of structures."

A simple sentence is simple  - a subject (usually a noun or noun phrase), a verb, and maybe an object.

• Smoking kills.
• Our climate is changing.
• Children learn from their parents.
• Growing old is not much fun.

Compound sentences use conjunctions such as 'but', 'and' or 'so' to join what could be two separate sentences, or 'independent clauses' in grammar-speak:

• Children learn from their parents, but they also learn from their friends.
• Our climate is changing, and agriculture is going to have to change as well.
• The population is growing older, so we need to spend more on healthcare.

Complex sentences have at least one independent clause, and at least one phrase, or clause. It can be as long as you like.

While growing old is regarded by many people as not being much fun, it can bring increased contentment, wisdom, and a more relaxed outlook on life, as well as hordes of grandchildren to play with and permission to do more or less what you want without people shouting at you or without you worrying what other people think. 

What kind of sentences are these?

Walking slowly along the river bank in the light evening rain with his chihuahua struggling to catch up to his heels, Detective Inspector Grainger looked like any normal person out walking his dog. He wasn't. 

Try to have a mix of short and long sentences -  simple and complex and compound  -  but try try try to keep the average number of words per sentence to between 12 and 15.  And please please try to

Topic sentences at the start of paragraphs can often be very short:

• Planning for the future has many advantages.
• It's easy to see why many people enjoy Hollywood movies.

Opinions can also be short

• A vegetarian diet is healthier and better for the planet
• Animal testing is completely wrong.
• In my opinion, children should be seen and not heard.

But there's no rule that says topic or opinion sentences have to be short. And don't use only short sentences -  your writing will be choppy. Have a mix.

Think of Band 9, and use a wide variety of structures!

However, I have learned that in an essay, there are three main kinds of sentences: simple, complex, compound. A good essay shouldn't have too many simple sentences which make the essay quite boring. However, I still don't know how to mix and balance between these three kinds of sentences. Can you help me with this problem, please?

Thank you so much! 

Thanks for this essay. As Brian said, it's well organized (two paras in the body, one about the fun of being a member and the other about the benefits in future), and there are not too many grammar mistakes, but the average sentence length is too high If you could add some shorter sentences, that would really improve it.

Brian found one or two small grammar points

a high proportion of students which are very inactive → a high proportion of students who are very inactive

Word Choice

Participating in clubs have become very familiar with students →
Participating in clubs has become very popular with students  OR
Participating in clubs has become very familiar to students

•  difficult-circumstance people → disadvantaged people/poor people/ less privileged people / people needing help/  people in difficult circumstances.

First of all, joining student clubs is one of the happiest periods of students’ life.

I wouldn’t use ‘joining’  -  I would change this to ‘Being a member of a club can be one of the happiest periods of a student’s life’

Shorten

An average of 20 words per sentence is too high. Some ways to reduce this are to add short sentences to reduce the average, break up longer sentences, reduce the number of clauses (which, that, whereas, etc), and keep sentences to one idea only.

Actually, being the member of clubs or organizations offers young people many special benefits that have been the fundamental advantages in their future jobs (24 words)

This could be rewritten:

Being a member of a club or organization offers young people many benefits and can be a great advantage in their future jobs OR

Club membership can have benefits for employment.

Brian suggested this change:

A club which its members coming from different classes forms a network which probably would still exist even after graduation

But I would really shorten it!

Club members can form networks which last/survive/stay together even after graduation.  OR

Club members form networks which can last/survive/stay together even after graduation.  

Conclusion

Try and be specific -  the 'take action immediately' is a bit vague. 

Each university and the students themselves should… take action immediately

Here's a possible rewrite which looks to the future but is a little more specific:

Universities should support the activities of student organizations, and students should make the effort to join clubs. Active campuses pay huge dividends for both students and colleges.

Overall, a positive and well-written essay, clearly organized and developed. 

Thanks nda_quanq, and thanks Brian!

Let me start by some figures http://www.online-utility.org/english/readability_test_and_improve.jsp.. Words count 318

Average number of words per sentence is 21 . I think it's a bit high.

Actually, your essay is well-organized and I cannot see considerable error in grammar.But I think it would be better we focus on the content whether it meets task response or not. I think if you sent the question completely we could better assess this, because you in some parts compared joining club with studying in school and in some part you are supposed that club and practical skills is same.

In the first paragraph you started with this sentence:

First of all, joining student clubs is one of the happiest periods of students’ life

Then I guess you should develop your idea that why this period is one of the happiest periods.But you are going to say that members of club are likely to be more closer than classmates. One of alternative would be like this:

First of all, it is more likely to find more close friends in a club student .

In the second paragraph you could add more ideas to support your opinion. But it might be argued that in terms of practical skills some vocational classes already give these skills to students and no need to take part in clubs. I think it would be better you focus on social activities that probably clubs give students.

A club with members from different classes and faculties forms a network that will still exist even after graduation

I think as Mr.Enda says avoid from generalizations.

A club which its members coming from different classes forms a network which probably would still even after graduation.

And the end of paragraph needs to add more details to support or at least put this sentence after you have mentioned that joining clubs give students experience.

In conclusion, students can receive many benefits while working in clubs and organizations. However, there are still a high proportion of students which are very inactive and always concentrate on theoretical studying. As a result, each university and the students themselves should notice the importance of taking part in social activities and take action immediately.

Id like to change this conclusion a bit to get easier to follow:

In conclusion, while students can receive many benefits from joining  clubs, there are still a high proportion of students  reluctant to take part in these activities. As a result, universities should notice  students the importance of taking part in social activities like becoming a member of a club.

Overall, I think your essay is very good and you benefit from accuracy. But I think you can improve your essay by clarity. Put more details after any sentences and use more relevant sentences. Thank you for this essay and sorry if I forgot some points and hope Mr.ENda helps us in this case

I notice you've posted three essays. They look good, but it takes people a while to read and to comment. It's probably better to post one and see what other people think. 

Then you can change your style or eliminate some common errors based on their feedback before posting another one. 

While you are waiting, why not post one or two comments (short ones -  they don't have to be long!) on this essay by Katiss or this one by Kenny, or this one on mothers as parents  by Shuaishuaish000 or this one by Nick-  or any essay you like in fact!

Thanks, and I hope people comment on your essay soon!

First of all, joining student clubs is one of the happiest periods of students’ life. While it might be argued that each student always studies in a big class with a lot of classmates, the truth is that not all of them could be his/her close friends. In contrast, the members in a club share many things in common such as goals, hobbies and characteristics. For example, all of the members in a volunteering organization are willing to help difficult-circumstance people and contribute to the society, so they do everything with their enthusiasm and solidarity among members.

Secondly, working during the academic period helps students learn many practical and useful skills, which are very essential for their higher education and future jobs. With the traditional educational system in Vietnam, students have little chance to learn soft skills such as teamwork skill, presentation skill or communication skill, etc. In contrast, the student clubs or non-profit organizations offer their members a practical environment that helps them improve themselves and promote. A club with members from different classes and faculties forms a network that will still exist even after graduation. Moreover, your experiences during students’ life play an important part in your CVs and attract the employers.

In conclusion, students can receive many benefits while working in clubs and organizations. However, there are still a high proportion of students which are very inactive and always concentrate on theoretical studying. As a result, each university and the students themselves should notice the importance of taking part in social activities and take action immediately.