@AkipuffRose I don't know what else to suggest here, other than to travel as a "vacation" to the University world and enroll once there. With NRaas Traveler enabled, your sim can travel to any world you have installed, and you can set the vacation length to whatever you want or use Pause Travel to get around the limit.
One thing that might help, but I don't have any confidence in it, is reseting the lot where the class is to take place, before class starts (so in this case, after your sim finishes the previous class but before the next one). Click on the ground of the lot and select NRaas > MC > Reset Lot. Any sims on the lot will be reset home, or to thin air if they're technically homeless, so move your sim elsewhere first if you don't want them sent home.
You can also have your sim study o the computer during the class to keep their grade up. It's not the same as attending but can be a stopgap measure if this problem is intermittent enough to not be worth starting over.