I really doubt that MC will get away with this. Teachers are not allowed to grade their partners' tests. Bella has even given and graded several of MC's tests, including the most important one: the mid-term test (she even protects him in case he cheats).
If B&R cancels MC's grades on all of MC's tests, MC could be in serious trouble. We don't know what the punishments are for students who are romantically involved with teachers, but they shouldn't be light punishments. In other words, MC and Bella are both screwed, so MC has a right to be pissed (or at least disappointed that Bella didn't trust him.).