Well a few years ago, as I was surfing the internet for some ideas, I saw one of those brilliant ideas that make teachers' lives so much simpler- a one size fits all crown!! I wish I could remember where I had seen it to give the teacher her much deserved thanks, but I guess this will do for accolades.
You will need:
Rubber band (!!)
Simply leave approximately an inch and a half at each end of the crown blank for the fastener. Have the kids decorate the crowns, and once they're ready to wear, the magic begins! Fold over 3/4" of one end, and place the rubber band in the flap that you created. Staple the flap shut so that the rubber band is being held securely. Now bring the other end of the crown around, and repeat the process, inserting the same rubber band into the second flap and staple shut.
Voila!! Your hat will now stretch to accommodate the slightly varying head sizes of your children! This is a particularly great trick for homemade class costumes where you'd want the hat to fit more than one child!