、。
You're students — this kind of thing is something you can only do now.
Literal
You-[plural-たち] [topic-は] students [explanatory-なんだ], this kind of thing [object-を] can do [nominalizer-のは] now only [copula-だ].
Advisory/motivational speech addressed to young people. 君たち 'you (plural)' is addressed to a group, polite but still a bit formal/paternal. なんだ frames 'you are students' as a reason or given. やれる is the potential form of やる 'to do' — 'can do.' ~のは...だ highlights a specific time ('only now') as the answer to 'when is this possible.' A classic 'seize the moment' sentiment delivered to students.