I was thinking the same thing as Rizzar Chriso.From a psychological viewpoint,when a kid makes something,makes it right,and it works as it should,he or she is extremely proud of it,thinks"I can do this",and sometimes,it can lead to a life long interest.Comparing a crossbow to a stonebow,I would think that they are much more likely to achieve success with a crossbow.Then there is the safety aspect,which is even more important.Others much more experienced than I on this forum have confirmed the safety concerns that I,and no doubt others could foresee.Personally,a stonebow is something that I might tackle after I had gained some experience building a crossbow or two.
As a teacher,I'm sure you are aware that kids have parents,especially Mothers!.Imagine this scenario.Kid comes home from school and says to Mum,"hey Mum,I'm building a crossbow/stonebow in shop...........Mum says "Oh,what's that?".Kid enthusiastically explains.Mum has a meltdown,and the brown stuff hit's the fan.Scenario 2.School phones Mum,and tells her "Wayne has been taken to hospital because the stonebow he's been making in Shop blew up".Mum grabs Dad's 12 guage,and goes teacher hunting.
I am sure you are a good and responsible teacher,and have your kid's best interests allways in mind,but getting kids to build weapons I would think is likely to be a hard sell,especially to Mothers,so building something that could be dangerous to the user,during construction,and use,where kids are involved,should give pause for thought.
Personally,I think what you are doing is great,and the skills and attitudes that the kids will get from building a crossbow or stonebow will be invaluable in later life,but going for the safer option(in my view the crossbow)makes sense. Best of luck,and success with your project,I look forward to seeing your progress postings.