"This is probably a silly question, but I was wondering
if there is some trick to keeping students from
tangling up yarn."
I keep mine in a carboard box with slits cut along the top. I run the yarn through the slits, leaving about a 10" tail on the outside of box. The kids pull on the tail and then cut; sometimes the balls roll around inside the box and have to be separated, but overall it works pretty well.
Marisa