Don't be nervous, the class is a lot of fun!
The first day is all classroom stuff, then the following two days are riding. The first couple hours of the first day is taught as if you were never on a motorcycle, so if you are familiar with clutch, brakes, etc it is a little boring (but do not try to advance past the instructors--they will not appreciate it).
I too had been riding on my own for about six months with a permit prior to taking the class, but I loved every minute of it and wouldn't mind taking it again in a few years to brush up on stuff.
Have fun and above all, follow the rules because they can drop you super quick if you are not paying attention.