Hi Shahab

Thanks for this Task 1 essay. Maybe I should open a separate area in the forum for Task 1, but I don't have too many examples yet.

Your essay is 185 words long which is fine, or perhaps a little long -  the requirement is 150. The sentences are a little long at 18.5. You could try adding some shorter sentences or breaking up longer ones.

Word Choice/Word Form/Usage

• accept my apologise → accept my apologies
• I invited them to my house to prepared our songs → to prepare
• he was year ahead   →  he was a year ahead 
•  they suggested to hire a studio → they suggested hiring a studio / they suggested that we hire a studio
• I hope this could satisfy you → I hope this satisfies you

Shorten

So we decided to hold a goodbye party for him and make him surprise. → So we decided to hold a surprise goodbye party for him.

Agreement

regulations which prohibits → regulations which prohibit  

I'm not really an expert on Task 1 in the General Training module, but it seems to me that you have done a good job of answering the question.

The only thing I would say is that there is some detail which could be omitted.  I also like to start with the main verb in the first sentence - in this case 'apologise'

I'd like to apologize for  OR  I am writing to apologize for the noise last night and explain what happened. 

I am a student in Calgary university and am in a band with some friends. One of our closest friends graduated last week and is going back to India, so we decided to hold a surprise goodbye party for him. As I am the only one who has a flat and due to dormitory regulations which prohibit playing musical instruments after 9 pm, I invited them to my house to prepare our songs for the party.

I deeply apologize for annoying you last night and I promise it will never happen again. When I described the situation to my friends, they suggested hiring a studio for a week and doing the rest of playing there. I hope this satisfies you and makes you feel comfortable again.

It's 140 words, but that's fine, when you add the greetings. But this rewrite has very long sentences -  an average of 20 words per sentence! Hmmmm, I'll have to think about it!

Overall, well done.