Power
Not a whole lot. The state decides the curriculum you need to cover. The school's principal tells you exactly the number of quizzes you need to give per week. And parents think they can tell you (and tell you all the time) that this word—no, this word—no, this graphic—no, now it's this chart—is offensive and objectionable.
You can't even get the kids in your health class to listen to what you're saying. They giggle every time you say the word "urethra" (which isn't even a dirty word), toss paper airplanes while you're explaining the menstrual cycle, and pull out their phones when you're trying to explain why pulling out something else (ahem) is a terrible idea. Does anyone listen to you?!