I’m so, so sorry that you are having to deal with this on a daily basis. You should absolutely not have to sit next to this guy and put up with his comments. No one, no matter how shy, should be subjected to such constant abuse.
I’m glad you tried to reach out to the teacher and principal. I’m astounded that they didn’t change your class or at least your seat. I’m even more astounded that this kid is allowed to continue making life hard for others.
What you do now is get off the Internet and talk to your parents. Withdrawing into the “net” isn’t going to stop the bullying. Withdrawing from other people by staying in your room isn’t going to help you get over being shy. Tell your parents what you’ve told me and ask them to intervene. You’ve done all that can be expected of a teen. Now it’s their turn to do what parents are supposed to do — protect you. They can insist that something be done.
If shyness continues to be an issue, please consider getting some counseling to help you learn new skills for expressing yourself and being more social. If you could do it on your own, you would have done it already. There is no shame in getting some help when you are in emotional pain.
I wish you well.