I really don't know what to say...
At the end of the day it is up to your son, the work he puts into school, wether or not he smokes weed at school etc etc
lol there really isn't alot I could say that you don't already know, just sit down and talk to him about it...be reasonable.
As friendowl said people use it for many reasons...ask him why, you might get a more accurate answer than asking us why he uses it...he might not even know.
....
I used to smoke a hell of a lot more than I do now.....and I noticed that it was effecting my school work...so I cut down to just smoking on weekends, and that seems to be working fine for me....
To be honest getting stoned in school is probably one of the worst things he (students in general) can do!

But yeah just talk to him and try to understand and see it from his point of view....hopefully he will be honest about it.



Edit: Good points mrdevious!