Learn Portuguese (Portugal): Vocês não são da minha turma, pois não?

Because the main statement is negative (Vocês não são...). When you want to ask for confirmation at the end of a negative sentence (like saying "are you?" or "right?"), you must use pois não?. If the first part of the sentence were positive ("You are from my class"), you would use não é?.

Why do we use da here instead of just de?

da is a contraction of the preposition de (from) and the feminine word for "the" (a). In European Portuguese, we almost always put "the" before possessives like minha. So, you are literally saying "from the my class".

Does turma mean the same thing as aula?

Not exactly! turma refers to the specific group of students you take classes with (your classmates). If you want to talk about the lesson or teaching period itself, like having a math class at 10 AM, you use the word aula.

Why do we use são with Vocês? Isn't são for "they"?