Use "the" in these cases: IF there are also students from other classes: 'the students from grade 9 class 2..' IF there are also other jobs/kinds of work to do: '... took part in the volunteer work' :)

Correct: Students from class 2 of grade 9 took part in volunteer work. ('Work' is noncountable, so don't use the article 'the' unless you are talking about a specific type of work as contrasted with other work.)

Either one could be correct. It depends whether it is all the students from class 2, grade nine, or just a few of them. If it was all, you should use 'the'; if it was just a few, you should say either "Students from class 2..." or "Some students from class 2...".

No it doesn't.

"Students from Grade 2 Class 9 took part in the volunteer work" sounds fine.