Natalie Munroe says she'll fight to keep her job in suburban Philadelphia.
She wrote that students were out of control, lazy, rude, disengaged, discuss drugs, talk back and complain about everything.
She did not identify the students, but some of them figured it out and alerted the school.
"I stand by what I said and I don't regret it because it's the truth," said Munroe. "And people need to take a look at what's happening in this district, in any district."
The school district is investigating and will decide whether to fire her.
Scores of online comments applaud the teacher for her honesty and supporters have started a Facebook group.