Im having an attack how do I calm my self for school?

My heart is beating fast I feel light and my hands are shaking. I am just scared about my grades. I usually have a's and b's and I have 2 D's in some C's a B and and A. I feel like I am going to flunk and I never had this problem. I also don't want to see the people at school. I wish I could do school online or something. It's going to take 3 weeks until I get medication for s.a.d. and g.a.d. I usllay miss school, this year I missed allot, but I have to go. idk how to calm my self when this happens I ussally just stay home, but I missed to many days