Need help

So here is the problem ive been trying to pick up a 4th As level to take and decided to try go for psychology instead of taking computing the head of psychology is fine with me doing psychology and one of the teachers whose class I would be in has said its fine but the other teacher (the class has 2 teachers) has said no but i dont know if she knows my gcse english (A grade)and science grades (B,B,A), the head of psychology has told me to go see her the teacher that said no (too late to join thats' what she said) and to try to persuade her to accept me in the class, Im also determined to catch up on the work we have a 2 weeks holiday coming up and plan to have caught up with rest of the class.

How will i persuade her ? should i take the head of psychology teacher with me and all 3 of us have a chat about if i can do it because if i go by myself i dont know if she'd let me even if i try my best to persuade her should i try to ask the head of psychology to come with me and talk to the teacher ?

really need help on this getting really stressed about it
apologies about any grammar mistakes

If you have permission from the head of Psychology, then why do you need the teacher's permission? The head of Psychology should be in charge of making the decision whether you are capable of catching up with the work. I would just bring the head of the Psychology to the teacher that said no to have a formal discussion.