I went to school high all the time in high school: Once before school, and again at lunch most days. I know it made me stupider, I just didn't care...somehow graduated with an A average anyway. Getting caught was never an issue, since I was an honors student and school security and teachers subscribed to profiling in a big way. I probably could have done lines in class and gotten away with it.
Now that I'm in college, I don't go to class stoned, although I have been known to get drunk in those annoying 3 hour long lab sessions that involve group work with people who you just know care more about the class than you do (thank you, sprite and hawkeye vodka).
Oh yes, and moderately off topic, but I interviewed the chief of the campus police while drunk for an article on binge drinking I was doing for the campus paper (PM me for verification if you don't believe me, the article is online).