How do I allow a student one more class this month?

On the student's profile, Subscriptions block, This month row: tap + ("One more"). The change applies to the current month only (0 to 99) and "Number adjusted for this month" is shown. Back to the usual number cancels it.

Both the owner and the instructor can do this. For a permanent change, the owner uses Edit > Attendance per month for this student. A class card has no months: it is renewed instead.