A level predicted grades

I got my Mocks back recently and did terribly. I got an E in Maths and basically another E in Chemistry. My school has extremely high standards for grades and most students get predicted A's and A*'s. What can I do to improve dramatically in the next 3 months? I can resit the exams in September before year 13 but I can't deal with getting bad grades again. I really need at least A in both subjects (I have one A in my third A level already).

Have you had the papers back yet? Do you understand where you went wrong?

I have got the papers back and have seen all my mistakes on them but I just don't understand anything in maths and chemistry. Maybe it's the fact that I don't like my teachers in Maths especially but I can't understand the concepts and can't even begin to answer the questions.

Which board is Maths?

There's plenty of youtube videos that teach every bit of maths and chem. I've personally found them way useful than my teachers and I only go to my teachers if Im struggling with a specific question. If you want that A/A* you need to put some effort in yourself outside of school and improve on yourself. Yes bad teachers can impact your performance but there are several ways where you can improve on yourself without a teacher such as using online educational resources. Maths and Chem is not a subject where you just learn and revise within your school hours, you should realistically be revising 4+ hours of each subject each week outside of your school timetable.

It's OCR B (MEI). I'm not sure how many people do this specific board but it's pretty uncommon I think.

What do I do about my condescending teachers though? I really like my subjects and am trying but I hate going to the actual lessons and end up not being able to learn anything.

Depends on the subject and if you're capable on teaching yourself. There's not much you can do regarding the teacher except work around them, after all you're not their only student. You can always request to switch to another class with another teacher too if its a growing concern, they'll probably take it on if you express how much you dislike going.
Especially since you're at the end of Y12, this summer is a great opportunity to really establish your foundation if you were to take a retake or even for your first Year 13 mock which is most likely before your teachers start doing your predictions (depending on also when you plan to submit your uni application). I know for my school that they heavily relied on the first Y13 mock we did for our predictions as we had the entire summer to prepare, and didn't look at my Y12 mocks unless there was a significant difference (eg an E to an A).
My maths teacher was extremely slow, not very good at explaining too but the way our course was structured (edexcel) made it easy for me to self teach myself the topics ahead of my class. I basically went to class to get my attendance mark and just did my own work while my teacher taught the rest of the class. Obviously it depends how your teacher teaches you, some don't like students being outside of their plans too much but another way to go around it is to just learn it at home and attend the class ready with questions. Also, I'm pretty sure TLmaths on Youtube covers OCR Mei maths- I've always watched his videos even when I was teaching myself an entirely new topic.
Hopefully you get a way better teacher in Y13, if it becomes a greater problem, talk to your form tutor or you're department head- they will actually care if it is affecting your performance

Thank you for your advice. Regarding speaking to department heads, I already have and they aren't letting me switch teachers so unfortunately I'm stuck with the same maths teachers till the end of year 13.
How did you go about teaching yourself the topics since I find understanding the concepts quite difficult?

hmm I don't see why they didnt let you but I guess its not my place to judge.
I can only speak for edexcel but for me it was a matter of watching videos such as from TL maths and thoroughly following his steps. If you don't understand even one step he does, then that's where you need to go and find what/why he did it, usually he does explain it quite well though. We also have a textbook that gives exemplar questions and a list of practice exam questions too. So after I watch the videos, I would then follow the examples. So I wouldn't just go blindly and attempt a question, but follow an already answered question that has already set out its workings first and see if I can follow it. If I can then that's great and I can attempt to do some questions blindly by myself. If I cant, then its back to Youtube, or even on TSR and ask someone for help. Theres also always some sort of pdf form online if your type up your question that someone has answered previously.
Maths always require constant practice. I've been doing maths almost everyday since I started in Y12 and the most I've gone without doing one was 2 days and that was when I was preparing for my mocks for my other subjects.

Thank you so much! I guess Maths will be my main focus over the summer now.