In a minute or two, the tension will spread up your arm. If you keep it a while longer, it will reach your shoulder. It may even start affecting muscles in your back. The usual cause of writing cramp is misplaced concentration. You tighten your grip to write better.
Sympathetic tension affects other muscles.
Soon your hand hurts. When this happens to a pupil of yours, begin by soothing the child. Remind him from time to time to relax his grip. This can take some time, but is time well spent. Let the child wiggle his hands.