As for getting good marks in school, it can definitely be done. I and this other girl I've smoked with before recently won awards for being among the top 50 students in our sophomore class at university.

And as for smoking and being a good musician, well, just take a look at the history of music over the past few decades. Think of Bob Marley and the Grateful Dead and the Beatles and Jimi Hendrix and Bob Dylan and the Rolling Stones and Phish and Louis Armstrong and Gene Krupa and pretty much every rapper ever.