With the stroke of a paint pen made for pottery (we used a Pebeo-brand Porcelaine 150 with a bullet tip), kids can turn an ordinary mug into a useful keepsake for their teacher. For an even more robust thank-you, include a gift card to a coffee shop.
To make a guide for writing evenly, place a rubber band around the mug.
Working on a folded towel to keep the mug from slipping, use the paint pen to write a message, moving the rubber band as needed. (Tip: Keep a damp cotton swab handy to wipe away mistakes before they dry.) When you're finished, bake the mug according to the instructions on the pen package.