Here's a link to a guide I made. It has the location of every collectable in the game, including all the Students In Peril.
http://www.xbox360achievements.org/forum/showthread.php?t=24...Unfortunately, unlike the other collectables there is no Red Brick extra that shows you where they all are. So just be thorough and make sure you've checked each area. The guide is colour coded so you can easily find each listing for the Students In Peril (they're green).
Unfortunately it seems a lot of people have trouble with possibly glitchy students ... all I can suggest is going back through all of them, maybe trying to recreate what you had to do to get it (if possible) or just do whatever you can and hope another one pops up.
According to Selkirk16, the student in the Boy's Bathroom remains stuck even after you save him. The rescued students usually run off. So if you're missing one, don't get confused and think that's the one you're missing!