It can mean a couple of things, and a lot of it you can tell from his body language. If he's constantly staring and when you look his way he looks away, it definitely means he's interested in you and just too shy to admit it.
If he keeps his unwavering stare he may be just trying to catch your eye, does he smile when you look his way? If he smiles maybe that's all he's trying to do, is smile your way and get some sort of recognition from you. Sometimes guys are too afraid to do anything until they already think you're somewhat familiar with them and interested in them.
DISCLAIMER: [The following may seem paranoid and over the top but my lifetime of training beckons that I make some mention of it. Considering this is a high school situation, the follow is *Very likely not applicable*
One thing to keep in mind when you have someone like this, especially when its someone watching you like a hawk like you describe it (paying attention to every little move) its not something to just ignore. This can be the behavioral signs of a predator, much like a cheetah stalking its prey. If he really freaks you out with the way he looks at you like this I'm sure that's normal, just having him stare at you.
If you ever decide to meet up with this guy and maybe try and get to know him, bring a friend or two along, it'll keep him from doing anything too weird. And if you ever notice him following you don't shrug it off. Keep walking and find some friends to meet up with or a group of people to chat with.
[End Disclaimer ;P )