I think I can share some experience from my teaching stint.
As a new teacher, I was put to teach 4 classes - a sec 1 normal academic (NA) class, a sec 3 NA class, a sec 5 NA class, and a sec 4 normal technical (NT) class. So eye-opening even the blind can see. I'll use the more politically correct term, these classes were challenging, and I must say that some students from these classes are good. As for the others... Some slept in class (which, in fact, is the least evil), some played cards, many took out their phone during lesson and played the games inside, some would run around the classroom, hitting one another with broomstick. Some would loiter around the neighbourhood after school, speeding about on their seat-less bicycle, or enjoying a moment of nicotine at the void deck. Did I mention those who openly stood up to challenge you, and stared you in the eye?
And then you may say, "your classroom management sucks!"
But do you know what powers do teachers have? The power to fail students' test. That's all. Detention? The guys were totally cool about it. Confiscate their ear-stud? They can always stick in a new piece. Confiscate their handphone? One got physical and then threw a chair. It is like answering to hostage situation armed with paper bullet.
Sometimes you try to talk it out with the parents, and what do they say? "Don't disturb me about my son!" "I'm busy". Nowadays it is very easy to skip school, all you need is a parent's letter, and some kids are good at signing for their parents. It is to the extent that girls skip school whenever auntie red visits, and boys skip when their outside business is good.