About failing a class and still getting into college I have already enrolled into.?

I just found out today that I cant drop a class I am taking and that I would receive an F for a grade if I no longer took it. I have already been accepted and registered for my classes at the University Im supposed to attend this fall. I am currently taking 2 summer classes at a community college and one I can not seem to get because since its the summer semester everything is crammed into a shorter amount of time and not taught very in depth. I was wanting to just take it at the University this fall and that way it would replace the F that I would have for stopping taking it this summer. My GPA right now is a 3.5 and I am worried that if I get an F could the University decide to not let me attend because of my lowered GPA due to it or since I have already been accepted and registered for fall classes with the University will I be ok? I have tried calling the University but no one is answering cause of the holiday weekend and I need some insight asap! Thanks for your help!