I was like that all through school also, if anyone was ever sick in class I would have to go home also as I would be in worse shape then the actual sick kid. In high school I started getting panic attacks in the classrooms especially if the door was closed and I was no where near it. It is truly awful not being able to relax and enjoy and just not be in a constant state of anxiety and worry.
I am sorry you are going through this, It has been over 20 years now since I was in high school and if it is any help at all nobody ever did get sick in class. Gr 5 was the last time it happened in school that I experienced!!