Hmmmm... Yep, you screwed up. Grin & bare it, babe. We do that if we fail to communicate. As the TEACHER you have to self control & think it thru, every time. Your kids depend on you. Failing is not as easy as "I'm sooorrrryyy!" Doesn't work.
Slap on the wrist aside, press on! I'm an ol' geezer with decades of dealing with art directors of multi-million projects. I expect them to come to me with definitive concepts of what I'm to achieve, budgets. time allowances, etc. If one comes up & says, "I wanna create a bar!" but has no designs, no nothings... You get my drift. You have to determine what you want the kids to learn. You have to set time & cost limits. You have to specify exact concepts to embed in their minds. You have to specify ease of material usage & cost. On & on. Your job. WE, on the other hand, get to sit back & help out as we can. But without your programming, our computers are worthless. OK? Figure it out, kiddo, & read up on the different materials. The Ultrecht site has some good but simple explanations of THEIR materials. Read, consider, ask Qs, purchase samples, test, decide. No quick answer here but good end results if you do the homework! B'sides, you'll have fun.
Gonna suggest that a moberator moves this to the sculpting portion of the forum so others can input.