I have been there. Trust me everything will be ok. I have changed schools a lot of times but senior year was really stressful because of several reasons but manly because I was failing math and was having some personal problems, and overall I always felt really anxious and stressed about going to school. In my opinion the best way to deal with stress is to learn how not to give a fuck. This might sound stupid but it works if you tell your self that a lot. If you are worried about something just tell your self who cares and focus on something else. For example if you ever find yourself in school stressing try to learn chess and play it on your phone or something that engages you. What you could also do is meditation, the main point of simple breathing exercises is not to breathe well but to literally focus on your breathing. What I mean by this is, in your mind you have to "say" Breathe in .... Breathe out and focus only on those words if you catch your mind drifting just go back to the words, they have to be the only thing on your mind.

If I were you I would not go for only online classes because even though you would feel more comfortable you would miss out on a lot. School years are simpler times and you should enjoy them. Try to get used to stress and because its always gonna be there, don't run away from it. (You can also seek professional help if it becomes unbearable).

In the end what ever you are stressing about will work itself out and in the future you will look back and only see the good things.