I used to use the 4 Ps (prompt, polite, prepared, productive) but developed more specific rules over the years.

1. Take responsibility for your own learning and complete all assignments with 100% effort. Put the work first.
2. Follow all directions given by the teacher.
3. Respect everyone and everything in the classroom.
4. Be on time and prepared to work.
5. Accept the consequences of your actions with grace.

I have a few catch-all rules there, but really rules don't matter much. It's all about the procedures. I couldn't possibly list all my procedures, but here are a few:

1. Bellwork is silent. Strictly enforced.
2. I have a conversation meter for appropriate time/volume/speech parameters at various times.
3. I have an attention signal (bell - I hate holding my hands up to get attention for a long time or clapping and I never make middle schoolers do it).
4. Bathroom is an open privilege (I'm pretty free with all kinds of passes unless need be clamping down) but I don't fill out passes. Students need to do that and sign out to go. Pre-signed passes available in one spot. I will conference with you and later call home if you're out too long (without cause) or go too much, no more than 1 person out at a time.
5. Hold up pencil to sharpen it if testing/I'm talking. Otherwise, get up if no one is at the sharpener (I have 4).
6. Silently throw away trash, get tissues, get supplies, etc, as need-be. Unless I'm giving directions or teaching directly. Then wait. Or ask if urgent (snot dripping or something).
7. Don't talk while I'm talking to the whole group. Ever.
8. Don't talk to anyone not in your group when working in groups. No off topic talk.
9. No phones out unless the "Using Electronics" sign is out and they are following the set guidelines for that activity.
10. Re-do any failing assignments; detentions issued for refusing to do work; mastery is expected.

I stress that all the procedures are about us being productive and the work and learning being NUMBER ONE at all times. Middle schoolers are social and I acknowledge this to the kids but tell them they're going to have to socialize elsewhere. We will cooperate. We won't sit in total silence much at all. We will laugh and maybe cry at literature. We will think about text. We will write our hands off. We will do all kinds of things. BUT we will not do anything that doesn't serve the work, and that includes anything needlessly off topic. That's my main thing.

I'm also very upfront with the "Hey, I can't make a rule for everything that could possibly happen, so if I say to do something, just do it. If you think I'm being unfair, talk to me privately and share your concerns - it may not turn out how you like but I'll listen (it's true, I will). If you mess up, apologize. I will too. I don't make rules without a reason but I'm not an omniscient God-like creature - just a person." Then I get to say what omniscient means, which is fun and teaches them a new word - they always ask.