My general rule is "Don't do anything that you know you shouldn't do"
Weirdly, that is not enough for many of my students.
I have the same rules that you would expect from all the other classes that ever existed:
Breaking stuff: Please don't break my stuff. Don't break pencils; I buy those. Don't pop the keys off the keyboards. Don't disrespect the materials that I have in the classroom - that's rude and infuriating. This is a hard line with me.
Disrespect: My students are 12-13 and they are in a spot in life where few things make absolute sense. They have lost the privileges of childhood and not yet earned the privileges of young adulthood. I have a fairly forgiving nature for the usual things: not sitting still, talking, daydreaming, doodling, etc.. I will lose my cool in a hot second over disrespect. Eye rolling, head bobbing, throwing your arms up, etc... I will call home quicker for disrespect than any other infraction. You may never hear a peep from me about tardiness, lack of pencils, or uniform issues - but I will text parents during class for disrespect.