Cute idea... good way to get them to learn square roots without them knowing it because they know the numbers under the post its! It is very inexpensive and not at all time consuming to make. The only problem I can think of with this would be the postit notes loosing their ability to stick onto the clock's face.
50 Things I'm Not Allowed To Do At Hogwarts. These Are Bloody Brilliant.:
The square root math trick is a great tool for anyone having to do numerous square root problems in school or work. Learn it on Curiosity.com and practice yourself: https://curiosity.com/paths/square-root-in-3-seconds-math-trick-tecmath/?utm_source=pinterest&utm_medium=social&utm_content=link&utm_campaign=2016q2xxpst03PTAYsquareroottrick