I let them run mad on that huge beach (the sea was way, way out in the distance) while the coach driver and I had our sandwiches. Suddenly to my utter horror, the tide had turned and what seemed like a tsunami rolled up the beach, engulfing my pupils.
The driver and I had several terrified children hanging on to our clothes, and we swam/waded back to safety.

Probably the tide had changed before you arrived. Contrary to the general impression, the tide does not ebb and flow in a smooth manner. It comes in at its fastest rate between 2 and 4 hours after turning. The details are Here

Not part of my general knowledge until a trip to the Maritime Museum in Jersey last week.