So I'm late to this party, but there's also a significant homeschooling movement in the US, which could work for you if the traditional educational norms aren't working for your plot.
Without knowing how it is in other countries, I don't think it has been mentioned, but everything in American schools happens with the bell. The school bell rings to start class, to end class, to start the next class. Of course, it's been decades since I've gone to school... If a teacher is absent, there is a substitute teacher in place. If no one shows, in college/community college, people would leave, in high school, some might leave, some would stay. Everyone would have to be in the next class anyway...rarely would this happen. Getting into college depends on the college, the applicant's grades, and the state they are in. For example, an out-of-state student would likely find it difficult to get into a California college. Ivy League schools would be hard to get into. At the other side of the spectrum, it's much easier to get into a community college. A student with bad grades, etc. would find it easier to recover their GPA there and move to a 4 year school for the last 2 years. There are specialty schools for electricians, mechanics, etc. I don't know much about them, though.
The teacher would get in trouble if he or she did that. There'd be a substitute teacher. But I had one American professor who didn't show up and there was no substitute. He acted very unusually. He vaguely described his past as having a lot of sex, drugs and rock and roll. There's nothing wrong with rock and roll, but he sounded very deviant and rebellious when he said it. Much of the older generation of that time considered rock and roll as the devil's music. Anyway, he easily got depressed, which is I think why he didn't appear one day. It's easy to get into a community college. If that someone still couldn't get into one, then he or she wasn't even trying.
