Another possibility would be a pair of cheap chinese ribbon mics. I've been playing with a pair of MXL R144 ribbon mics, which I like very much. Obviously you would have to train your students to use them without damaging them, but the figure 8 pattern and strong off-axis rejection would make them good for teaching purposes. (Blumlein configuration, MS)
One thing I learned was never to buy them used. This led to learning how to replace the ribbon.