I like a good slasher movie as much as the next person, but from the outset, this one is very formulaic, slow and predictable.
The scene is set some years earlier with the baddie killing the heroine's family. It's briefly explained that he was an obsessive teacher but doesn't really go far enough to explain why this chap is so intent on killing the object of his desire.
The film moves on at a slow pace, eventually centring around a high school senior prom, with the typical characters of a super bitch surrounded by an adoring entourage, in pursuit of being Prom Queen. The Heroine and her friends are of course not part of that group.
The man escapes from his psycho ward, finds the prom and starts killing people in a frankly, dull way for a slasher movie.
I'll not spoil the ending other than I was glad to get there.
The basic concept of the story is fine, but it could have filled out in a far better way, giving us a good understanding of this teacher and his obsession with perhaps a vengeance twist against the school, staff and students. This isn't a film you need to try and think through, which for me is probably what lets it down.