As far as I know, there have never been individual enrollment classes in May. Last year they were in January-mid February, and mid June-October. This year's dates look similar.
Our tickets were valid 7 days prior to the arrival date we gave during registration, and for 7 days after the departure date we gave; they were not in any way tied to the date of the class. You won't be able to get into the park for your class without your tickets. As others have stated, there are definitely ticket windows open later than 2pm.
If you register for the class, pay for your tickets, and they cancel the class, your tickets will still be good. They will try to help you reschedule, but if no other dates work for you, then you won't be penalized.