Most Helpful Girl
There are many possible reasons why he quit looking at you, or "lost interest".
First possible reason: He probably wasn't interested in you in the first place.
You made him feel good by complimenting him, and he decided to return the favor by paying attention to you.
Second possible reason: He probably has a girlfriend. Maybe he thinks your cute and wanted to flirt a little, or maybe just enjoyed looking at you but never tried anything because he has already has a girlfriend.
Third possible reason: He's probably really shy, or gay...
Fourth possible reason: He's just not that into you
If you really want him to notice you, you have to actually talk to him, and quit playing stare-tag. After class, or when you bump into him in the hallway, ask him a question, or a favor, or make a comment on something (ex. Nice shirt) etc. DON'T ask him out, or ask if he likes you, or ask for his number, or his hand in marriage... Just be cool, but flirty. If, after a couple of days/weeks, he still doesn't show any interest.. then move on =)