The most frustrating thing that happened to me as an instructor, was to have students who come when I'm trying to set-up the classroom, and expect me to help them pick out fabric and cut it for them. My set-up time is important for the whole class. A student who comes un-prepared, and expects everyone else to wait while they take the instructors time, is not alright.
My best classes are ones where I can get there half an hour early, and get set-up without students needing my attention until class starts. I love it when students bring what is on the supply list, and have come to learn. I understand that for some, going to a class is like a girls night out, and they want to socialize. But don't monopolize the time with chit-chat. Other students are there to learn, and can't if the teacher can't get a word in.
Class is not the time or place to air your dirty laundry or talk about controversial subjects (student or instructor).
Students and instructors both need to be patient. Some students have more cognitive abilities than others. Some will need more attention from the teacher or from other students. It's great when students help and encourage each other.
Don't come if you're sick enough to get someone else sick.
Be comfortable, ask questions and have fun. Treat others with respect.
I try to bring chocolate and water for everyone. And it's important to have a professional, clearly written handout or pattern, and know it's every detail.