Experience is important. Experience with discussion and reflection is even more useful. We learn a lot more from failures than successes. So I say protect the kiln, and other students pieces, and give them as much experience as you can. "Shelf of shame" is an interesting concept. But I prefer more the cause and effect connection. There are pieces that derive their beauty from over glazing. The artists may have to work the piece a bit and have a bit of cleaning to do, but the results can be stunning.