I think this is very unorthodox.
It's a reading week so they're getting to do stuff "off topic" according to the previous newsletter although unsurprisingly there was no mention of using pants on a string pegged up and stickers on the gussets to make sums 
And wearing of pants on head as a prize?
Inappropriate!? I hope they're not second hand pants. My dc spent most of the morning tearing their way round the house with pants hanging off them, they had to be frisked on exiting the house for fear of them packing supplies to take to school.