Her logic is kinda good, she knew he had more than enough advantage to defeat him without cutting off limbs, so a punishment is reasonable...
But on the other hand I recall a previous chapter when all the academy's staff was requested to urgently gather just because MC wanted a day off, so she should've been aware that MC was a big deal.
She and the principal should have known better, now I guess the academy will be forced to issue an apology, the teacher will be pissed off because she lost face and will do something stupid that will end up killing her, not to mention the idiots who are scheming against the MC.