Several things.
1. He should've gotten mad that they drag him into this, not because they just cheated. This manga has shown the potential to do away with the goody two shoe trope, follow through like how the MC was still wary of Takae teasing him.
2. What kind of a fucking joke of a class is this that the teacher leaves teenagers to take a test unsupervised when some are already suspected of cheating? Was the punishment just to make them show up?
3. The girl wasn't being an air head in the end, you were just being nosey. Keep your damn eyes on your table.