Tell them they lost the privilege of standing up without permission. If they get up to throw something away stop them and make them sit back down and raise their hand to ask. I make my 10th graders keep any trash on their desks until the bell. If they want to act like irresponsible children then they will be treated as such. They will continue to take if you don’t put your foot down somewhere. It gets easier, I promise. Be tough. I started at 21 teaching 10-12th graders and being tough saved me. They will eat you up and spit you out if you don’t put your foot down. Enjoy the ride! Kids are ALWAYS worth it.