A drug dealer, usually in this case a student at a college or university that is dealing in addition to their studies. However a Chemist on campus may not necessarily be a student, possibly just someone who hangs around targeting students.
Person A : 'Hey bro do you know if there is a chemist on campus? I need to reimburse my supply as I got high last night with my cat and used up all my skunk'
Person B : 'I think that Johnny from our algebra class is, you should go and ask him'