How to not freak out before a test you know your gonna fail

Accept that you are going to fail.
It's fact you can't change it.

Just do the best you can do and get as high a mark as you can you may surprise yourself.

I don’t think I can. What’s making me worse is that there’s a entire class around me also knowing they’re gonna fail. When your teacher can’t even answer the questions you know they’re hard

What is the question?

I've known people who got a scrapping grade in maths because they always showed their working out even though they got the answer wrong so being wrong and failing isn't always the same.

It’s multiple questions. There’s questions she tells us that will be on exams that even she can’t answer. If I listed them all I’d be here for a hour. But they’re usually ones with multiple parts and they have to be done perfectly or you get 0 at least in my school

You're doing mocks though right? If that's the case then don't worry about it in the long run they don't matter.
Also if your entire class fails like you think will happen chances are it's your teacher that will get in trouble, not you.