Does anyone know if its possible to pay on princple only on a school loan. If my loan lets say is 30K and I want to pay in 10K lump sum....will it bring it down to 20K or will it just all go to interest.?

I would hate to pay like 20K on a loan of more that 30k and find that it all went to interest. Does anyone know what will happen if this is done. I can't even trut paying the amount in full if I even had the entire thing cause I would be afraid I would be a penny of and give them the opportunity to put it all towards interest. School loans are the work of the devil and federal school loan laws should be much easier to deal with than they are. Please let me know if anyone knows about this issue. I am currently going on ssi due to a disabling back, but i still can't get a doctor to fill the papers out correctly for a total discharge so i will end up having to use any benefits to help pay on my loan debt.