I disagree with the idea that volunteer work should be mandatory for students, and I have two reasons to support my opinion.

First of all, volunteer work shouldn't be forced. Even if it's a good experience for students, they should be able to spend their private time for doing something they like.

Secandary, Secondly, if the volunteer work is part of study, it will reduce their study time. Students should more focus on studying core subjects in classrooms to improve basic academic skills.

For the above reasons, I disagree with the idea that volunteer work should be mandatory for students .

When we speak or write generally about a subject, we usually use the plural form (students).

The correct structure here is 'spend time + DOING + something' (I like to spend time walking in the mountains).

Here, the adverb 'secondly' (meaning: in second place....commonly used to introduce a point or reason) is needed instead of the adjective 'secondary.'

You need more examples here. e.g. Students should more focus on studying core subjects in classrooms to improve basic academic skills.

"part of their study" is slightly unclear, you should use "part of their course" or "part of their curriculum" or "part of their studies" to make the sentence clear.

'Above reasons' are always specific reasons already mentioned, and we always preface this with 'the.'

Your two reasons were backed up by logic, and you had a strong opening with a solid conclusion. When possible, try to add some more details and examples.