The secret is...
Use a FLEXIBLE mixing bowl so that you can do this when the plaster is completely dry
[watch the video below]
I tried scouring the local thrift stores for flexible mixing bowls but I actually found the best deal for them at the local dollar store!!! I would recommend when you find one/some that you think might be worthy of buying for your class, try and flex them in the store the way I demonstrated you do it to determine if they are worth the money. I mean... the dollar store isn't expensive but buying enough bowls for your class aren't cheap! I have had a few bowl end up cracking and breaking over time but it's been no more than a $3 loss probably so that's a pretty good deal.