You may not have had a problem with these guys, doesn't mean other people won't. Were you there to see him show up to class late or getting drunk during the middle of the day. If you just heard it from other people (for example people at the school who may have wanted to get rid of this teacher for other reasons such as age, looks, not getting enough profits because students don't like his/her teaching method, etc.) you may have just been duped.

You can find people who have worked for a couple of years at some of the worst companies on this planet and still call it "nice". Doesn't mean it actually is.