* Put scotch tape over their bobbins. This will make it difficult for them to thread their needles, and they'll be frustrated trying to figure out what's wrong.
* Sew their project together. This is a more elaborate prank, but it's sure to be a memorable one. Sew the teacher's project together so that they can't finish it. They'll be furious, but they'll also be impressed by your creativity.
* Put a fake spider in their project bag. This is a surefire way to scare them. When they reach into their bag to get their supplies, they'll be surprised to find a fake spider staring back at them.
* Give them a gift certificate to a yarn store. This is a nice prank that will show them that you appreciate their passion for crewing. They'll be thrilled to get a gift certificate to a place where they can buy more supplies for their projects.