Exempt them from those performances. They didn't pass their English class, why do they have to do more for their band class? Why do you *want* to do more reading/grading assignments? Let them focus on the classes they need to pass.

By law, they cannot attend the event, so they are doing what they are required to do. If they're continuing to attend rehearsals and participating in class, there's no need to make anyone's life harder.