In a ceramics class, after demonstrating and assigning a piece to work on, I always add the following caveat: Your work can not be drug or sex related, as taxpayer money is funding your art. Should you feel the need to adventure into those areas, then please do so in the privacy of your own home. You inevitably get those students who want to make "bongs"-or waterpipes, or some such nonsense out of every project in art, with the "Oh Mrs. H, it's not a bong, it's a lamp!" So after very little discussion, as I tell them I think I have made it perfectly clear, I let students proceed through the entire experience of working in clay, while watching that they are considering it's usefullness in the drug-related area, whether pipe, or bong. Then in full view of everyone, I pull my "ole silly me, I dropped your pot on the way to the kiln routine." It works, and again, I have only had to do that once in 20 years. The student got a grade for construction, creativity, use of materials, but oops, it didn't survive! Sorry!
Cruel, maybe. But it's effective for me. And never any complaints from students or parents.