Glad I have you thinking about it. It all depends on how you want to have have the whole thing set up. If you set it up that way, and a class fills up, the next person who comes along will not be able to select it as a choice. If someone later decides to drop the class it would be available to the next person who goes to sign up, which isn't exactly fair, but if that's not a concern it would be a much simpler way of doing it.

Another way would be a variation of what I mentioned above, quering the comboboxes. Setup the comboboxes so that only one of them is enabled at a time. So that when the form is opened they can only select combobox1, once they make that selection, it will enable combobox2 and disable combobox1. This would extremely simplify any queries or if statements. You could also create a clear button that resets all the comboboxes so they can change their choices.

I'll have to get back to you on the rest of it, I've been busy working on other projects at work. I was hoping someone else would pitch in.