Some life lessons that teachers teach aren't officially part of the curriculum. Being human, teachers sometimes snap at their students, or roast them sarcastically to assert their authority. These teachers weren't afraid to sound mean while giving their students constructive criticism.
The note says:
Sometimes I go home and can't sleep because your annoying voice just doesn't stop running in my ears, like the worst torture known to man.
Than is all.
Ash confirmed the veracity of the note.