Right, you choose the number of classes as part of the enrollment dialogue.
You could go back to the main menu without saving, reload the save, and then enroll again. You should have the same scholarships that way but that will only work if you saved after the scholarships were awarded and haven't saved since you enrolled. Another option is to recover an earlier save that was made before you enrolled but after your scholarships were awarded. If you don't have any saves that meet those conditions, you'll either have to play through the term or withdraw in order to add more classes.