If you're dealing with a high schooler, they are just trying to be a 'cool' and make themselves feel like they're a badass. Chances are they know you're not doing drugs, they just thought it was a funny thing to ask you because they're not mature yet. I had some real weird questions about my stutter in school, too. Unfortunately some adults are just as stupid about it, but I haven't had an adult accuse me of taking drugs when I stutter.