A fidget toy that mimics bubble wrap - without the loud popping noise - is the latest craze to sweep schools.
Demand has been so huge that some retailers regularly sell out of the toys, called Pop-Its.
Made of silicone, they have lines of bubbles that children press in and out, and come in all shapes from round and square - to unicorn.
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Principals' Federation president Perry Rush said, after three decades working in schools, he still found toy trends unpredictable.
"I can't predict the next big thing, but one thing is certain - there will be a next big thing - they come along very frequently. It's always interesting to see what captures the imaginations of young people."
The popularity of fidget toys was unlikely to be a result of relieving post-lockdown stress, he said.
"Fidget toys come around in cycles as do fads with young people. In my day it was marbles that captured the school population. It's currently fidget toys and fidget toys were in vogue a couple of years ago."
Most schools encouraged children to keep toys at home, Rush said.
"Some young people need to have active hands to help them concentrate and teachers make decisions locally about those children and whether or not it's appropriate to have a toy that might help them to concentrate."