It sounds as if you have some phobia going on here - be it agoraphobia, social phobia or school phobia. Have you spoken to your therapists about this? Also, when this is happening, think about and note down what is going on in your head at the time to be able to explore with a therapist- it might aid them in their understanding around why it is that you react like that. For example, you are about to sit in the car, you feel your heart pounding, you feel sick in your stomach and think "oh no, not again, why does this happen every time I go to so school" and "oh no, everyone will notice if i get sick at school or make a mistake" or "i;ll look stupid and make a fool of myself" etc. Those are thoughts in line with social anxiety disorder; with CBT you would explore them and look at different ways of thinking so to no longer have those thoughts and eventually in time reduce the SAD. I know it feels difficult at the moment and looks quite bleak when you are in that situation, but eventually it'll work out. Source of knowledge is based on my Clinical psychology studies - although I am not a clinical psychologist and this is advice, not a diagnosis, etc. :)
And I wonder about your statement that "nothing helps". I would think the school's accommodations - that you can eat lunch in the nurse's office and not be called on in class is helping a LOT. Anxiety over who to sit with at lunch, and being called on in class are major stressors, typically. Are you saying that if you didn't have those accommodations, your school day would be exactly as difficult? If you had to speak out in class, or had to find friends to eat lunch with, you wouldn't have more anxiety?
Stefie, you're brave and tough. Anxiety is HARD, and yet you realize you need to graduate and you usually go to school despite your anxiety.
Do you have an exercise routine? Even walking long distances outdoors is really helpful (treadmills indoors is less helpful). Do you purposely do things for pleasure - play music, play with pets, dance, etc?
Best wishes. You're tough, and that counts for a lot.
Does your therapist specialize in anxiety treatment? Most of them don't. I'd also want to investigate further the fact this seemed to start when you started to have your period. It could indicate your problem is a hormonal one. Have you ever seen a doctor for a thorough work-up to eliminate all the physiological conditions that can cause anxiety, including hormonal imbalances, thyroid problems, blood sugar problems, hidden viruses, nutritional deficiencies, food allergies or intolerances -- heck, it could be a dairy or wheat allergy given how young it started for you. Keep seeking out what woks for you, your youth is your strength.
Did they take blood tests to check your Vitamin D and Vitamin B12?
Magnesium is helpful for anxiety as well.
Many medications hinder absorption of vitamins/minerals and if you're undiagnosed and then given prescriptions that might further deplete levels.
If you are low...raising levels can really help.
Sometime lavender is very calming, also look up pressure points online. You can do this to yourself.