Stay vigilant!
Google for suicidal tendencies in people.
I tried to commit suicide myself once and I can tell that suicidees are quite, reserved and don't talk about their problem to anyone. What drives most of them is that they have no one to speak to about their problem. Take notices of all nerds that get bullied all the time or of sobby and melancholical girls that recently got dumped by their boyfriends or vice versa!
Remember that you can't pull this on your own. Organize a team, work in correspondence with the school staff. The school psychlogist needs to find such problems, but thing is, neither you, nor me trust the school psychologist, because we barely know the guy. So get your classmates to talk with potential suiciders, ask everyone if there is a problem about which they wish to speak about and do not forget to mentiont that you are trustworthy.
Hopefully, this will help.