I guess there are crappy teachers. Just like there are crappy doctors, car salesmen, dentists, farmers, mechanics, police officers, electricians, cosmetologists, lawyers, nurses, governors, senators, presidents, carpenters, principals, bus drivers, plastic surgeons, bartenders, pizza delivery guys, taxi drivers, etc.
The difference between the government teachers and most of the other occupations is that with the others, taxpayers aren't forced, at gunpoint, to pay them even if they don't use them. If they want to use another lawyer or pizza person, or not use a plastic surgeon, they are free to do so without penalty.