Can I attend classes for more than one instrument?

Yes, if you sign up for a mixed instrument class; otherwise not. This is so class sizes don't get too big, and teachers can get to know their students and build each day on what was worked on the previous day. 

The rest of the time, during the jams, elective classes, and office hours, you're free to participate with any instrument you want, and work with any instructor you want. So, for example, if you are in a guitar class in the morning, you can take an elective class for mandolin in the afternoon, and play banjo at an evening jam session.