1.I firmly believe that this policy can provide several advantages for teenager(s) themselves as well as the community.
2 Instead of spending their free time relaxing with family and friends, these (?)adolescents have to work long hours without receiving any incomes
(Forcing the youth to work during their holiday can have a negative influence on their mental and physical development .you did not mention anything regarding the payment in the first place, then "income" just pops up ..hmmm...just feel a bit strange.)
3.some companies or private organisations
( i prefer to write like Just " some companies" or "some public and private organizations/sectors" )
4. teenagers are sometimes required to carry out heavy work which might not (be)suitable with (for)their physical abilities.
(which might not be suitable for their physical abilities. still sounds very odd...)
5.However, there is no denying that this strategy can give valuable benefits for both adolescents and the society
6. In addition, such voluntary work provided by adolescents plays an important role in the development of the society.
members can contribute themselves to assist disadvantaged people or those who have just recovered from natural disasters.
(well, as far as i know, contribute to +noun is the more common use of this phrase. hmm..although contribute to +verb-ing is not incorrect but sounds ....the same.....unnatural ... just my own opinion ).
8 As in you state your opinion in the first paragraph, would it be even better to reiterate your opinion in the last paragraph?