Silly me! I thought that the new fence erected all the way round the lake in the school grounds was to keep the students out (to prevent them falling into the water or venturing on to the ice in winter). However, the sign shows that the real reason is to keep the inhabitants of the lake in.
Can't think how I got it so wrong.
Incidentally, the fence may well prevent the use of the lake as a resource in lessons. Biology teachers used to go "pond-dipping" to examine all the little creatures and plant life in the water, but that is probably out of the question now.