When a troubled young man on community service discovers that a teacher isn't all she seems to be, he needs to stop her malicious plans before she gets a hold on the man he loves.

Yes, these descriptions are quite vague

"isn't all she seems to be"
"stop her malicious plans"
"gets a hold on"

I disagree with the others re 'community service' -- I think this can be the hero's ordinary world in the beginning, but the other elements of your logline need to relate to this world somehow, i.e. the antagonist is a supervisor, not just a teacher; the hero's action involves escaping or uncovering corruption or stopping a murder etc within this closely supervised world; or because he is a convict, if he discovers something bad, no one will believe him because he is the 'turd in the rose garden'.

This is super vague ...

In what specific way is he troubled?
What has lead to his community service - and if it's not relevant, don't include that he's doing community service.
What sort of teacher? What does he discover about the teacher?
What are the teacher's plans?
Why does your protagonist do in response?

All I know right now is that a guy on community service finds something out that could put his lover in danger. There's the barest bones of a concept there and a statement of the stakes, but not much else to go on.

What are her malicious plans? What does "gets a hold on" mean?

Also, presuming the troubled young man is the protagonist, his goal, such as it is, is stated negatively, "to stop her" yada-yada. What is his objective goal stated positively?