I second the call for Tk, to teach beginners. It is easier to learn and get up and running than any other gui. You do a dis-service to students when you start them off writing with a GUI designer. You will spend more time just trying to decipher the boilerplate code those designer programs emit, than if you just taught the basics of windows, bindings, and event loops.

If you are under orders to teach with a GUI designer, Glade is still probably your best bet.

If you notice, very few people actually use those GUI designer programs for anything more than simple popups and displays, because it just gets too hard to follow the boilerplate code.