If I could send a letter to my younger self when I was in grade 7, this is what I would say.
I am you in the future, I am 20 years old now and attending my second year of college. I know that you never really thought about school after high school but it is much better. I know right now you are very discouraged with yourself but trust me (or yourself rather considering we are the same person at different points in time) we can survive this. I know doing homework seems pointless right now but try your best to attempt your math homework. Your teacher right now may seem like an idiot and an evil person, I can tell you that you are absolutely correct. The way your current teacher reacts to your math questions is not right. It may not seem like it but you are not stupid for not understanding math, you just need more help than your friends. Eventually, you will get the help that you deserve. You do not deserve to be treated this way be teachers that are supposed to be there to help you, not ridicule you. If you take anything from this letter, please remember that you are worthy of help and friends. You are going to do great at life and meet lots of awesome people.
Have a great year,