Your expiration date on OPT is the date your F1 visa expires. You don't need another I-20 to finish your class. Even you are in OPT, you are still in F1 visa. Your I-20 now has nothing to do with anything until your OPT has its validity date. Your school needs to let your attend that class without the need of I-20 as your OPT is the prime object for your student status in US.