Extension of Student visa question

Hello. I'm an American student here in the Czech Republic with a long term stay (6 month) student visa. I got it in September and it expires in March. However, classes end in May.

How would I go about extending my student visa? Obviously, I'd need confirmation from my university. When getting my initial visa, I bought insurance for a year, so would that insurance suffice? Also, the document I initially submitted for accommodation was for all 3 years of university. Must I submit a new accommodation document, even though I've registered at the foreign police and my address will remain the same?

I'd greatly appreciate if anyone could help me.

Thank you.