We always line the young children up after break and lunchtime to visit the toilet. It does, however, seem a bit unnatural to me to expect every child to go for a wee at the same time, but it seems to work. It's also a good opportunity to wash hands after playing in the mud, sand etc. Any child who asks to go within the next hour is frowned upon, but not denied access to the toilet.
I would enquire what the class policy is. At this time in the school year teachers do seem busier than usual, so maybe the usual routine has slipped a bit?