I'm a retired music teacher. I assure you, band class is graded and elective credit given out. In my experience, there is no distinction between 'marching band' and 'band'. I suppose there might be some schools that do not have that as a policy. Best to check. 

As for being excused from performances, that is up to the individual teacher and his classroom policies. If the teacher gives an alternate assessment, a student can receive a grade and credit even if they are not there for a number of performances.

If the class receives a grade and credit, it is co-curricular.