Teacher of 18 years here, grades 8, 11, & 12. I find that the first 2 days and the first 2 weeks set up your whole year in terms of discipline. Take the time to explain the rationale behind your "class rules", be fair, firm, and friendly when STRICTLY enforcing these rules the first few weeks of class. Another thing I do on lecture days is give 2 minute talk breaks half way thru class. Let them get up, walk around, talk to friends. Use a timer that will beep. Set it at the beginning of class and when it beeps, stop. Then reset for 2 minutes and the class becomes conditioned to respond to the timer and actually get seated and back to business quickly. All of my rules have codes and I give everyone one freebie. When they violate the same rule twice they have a detention after school. It is imperative that when any rule is broken you call it out and document it, in a nice way....i have a pocket size "black book" for this task. My detention involves the copying of a randomly selected page out of one of my engineering or physics textbooks from college. They can bring a friend for company. I have groovy tunes going, and they can spread the copying task out over several days if they want. I also give them 2 weeks total to complete the detention. I've yet to have a case where a student did not serve there detention. Oh yeah, it is possible to pick a blank or mostly blank page in which case the detention only takes 5 seconds. It's rare but the choosing of the random page turns into a "lotto" where winning is choosing an "easy" page to copy. The most important part of the entire system is to document the violation as quickly as possible and get back to teaching as if nothing happened. Good luck!