On page 4 of this document it explains a little

http://www.cdc.gov/concussion/pdf/TB...HERS-508-a.pdf
Dept. of Heath and Human Services.
Fact sheet - Concussion

Gist of it is - reduced time for certain activities (computer, reading), rest breaks, more time for tests etc..

It depends on how you're doing and how much the teachers or school personnel understand. Sometimes, teachers can be more "with it" and put these accommodations in place without a Plan, but sometimes the parents or doctor or whomever need to get involved and sort out the legally binding 504 Plan with the school to make sure the accommodations are actually being made.

I had a few others, but they were extremely long and didn't want you having to read it all to get the info.. So basically this is just an example. I'm surprised the teacher/s haven't already put accommodations in place prior to semester starting for you.

BTW, it's only just over a month since this 2nd concussion, so you still need to take it easy as you can.
you will get better. You just need to ease into things slowly and not when you have symptoms.