Why's she so bad? She's really mean, discriminates against werewolves, treats Harry like dirt and at one point sends a Dementor to attack him and Dudley. Bad lady!
Why's she so bad? She's been teaching second grade for 300,000 years, which has left her absolutely horrible. She hates Gumball and regularly lies, cheats and goes out of her way to make life more difficult for him. BOOOOO!
Why's he so bad? While Yoda's really lovely and everything, his daft way of speaking would just make being in his class really annoying. He'd mess with your emotions by saying things like "Pass your exams, you haven't", where you'd get all excited at the beginning then be really let down. Awful, it'd be.
Why's she so bad? She treated Miss Honey horribly, and possibly even murdered her dad! She's fond of punishing wrongdoers way out of proportion to what they've done. She's dead good at shot-putt though.
Why's he so bad? Duh, the clue's in the name! He was a wrong'un when the first book was published in 1982, and a new book came out this year – that's 35 years of being a hypnotisey wrong'un!
Why's he so bad? He's not so much bad as totally rubbish – he means well, but is just sort of clueless and pathetic, which isn't his fault, but probably doesn't really make for the best head teacher ever...