The short answer: have your students set their own rules, codify and display them on a piece of poster board, and hold them to those rules.
No matter how rude and rowdy they seem, once they get to a certain age, students know how they should be behaving. If you have them come up with their own rules, they will feel more inclined (and obliged) to follow them.
“Listen to whoever is speaking” and “Only one person can speak at a time” are commonly suggested rules, but you may want to have a discussion about whether or not to institute a “Raise your hand before you speak” rule because it might disrupt the natural flow of conversations. Another gray area is the issue of criticism. Students may want to prohibit any form of criticism from being voiced if the topic at hand is particularly sensitive (i.e., in a sex ed classroom, students should be encouraged to be unconditionally supportive of everyone’s views and opinions). But students might also point out that disagreement is normal and an integral part of intellectual discussion, in which case they can come up with a rule about presenting criticism in constructive terms. Another rule you might want to have your students consider is confidentiality, if you plan on having them share personal stories in the classroom.
What are some of the rules you’ve had in your classrooms?