Allow for "Instructor TBA" for a class on the schedule

When we plan our classes a few weeks in advance, we don’t always know who will teach a certain class. It would be neat if we could select something like “Instructor TBA” instead of having to select one.

This is a nice to have. Definitely not a priority item. The workaround is to create a fake instructor account and name it “Instructor TBA”. Perhaps that’s good enough.