I'm in high school and my high school doesn't celebrate teacher's day.
This is my second year of high school and last year I had this really nice teacher that I really liked
But last year I didn't write her a thank you card, She is not my teacher this year but she has helped me with some school stuff. Would it be awkward to make her a thank you card. I feel like its going to be Awkward because she wasn't my teacher this year and because my school doesn't celebrate it.

Also I have anxiety and I might have a panic attack right before giving her the card so do you have any tips on how to calm down when having a panic attack??

I think it would be OK to give her a card. Just give it to her and walk away. There is never anything wrong with letting a person know you appreciate them

Does she have a mailbox at school?

I often get notes, or small gifts from students, a card is very proper to do. You can also just leave it on their desk. But no need to say anything.