They are marketed as a stress-reliever to help children with learning difficulties concentrate in class.
Yet fidget spinners - the new craze sweeping playgrounds - have instead become such a classroom distraction that the hand-held toys are being banned in many UK schools.
The gadgets consist of a three-pronged, palm-sized piece of plastic or metal which spins around a central weighted disc - a modern version of the old spinning top.
They can cost less than €2 but deluxe versions change hands for nearly €50 and YouTube videos demonstrating how to do tricks with them attract millions of views.
Chris Hildrew, a headteacher in Somerset, shared a letter from one pupil complaining lessons were being disrupted.
"They are the latest craze and roughly seven people bring them into my lessons and share spares with other people," the unnamed girl wrote.
"When you are trying to focus on your work, all you can hear is it spinning round and round.
"If someone around you has one you kind of get attracted to it because they are trying to do tricks and everyone else is looking at it. This means that I am not doing my hardest on my work so I get less done.
"To sum up, I think they should be banned in lessons."
Mr Hildrew, posted a copy of the letter on Twitter and wrote: "We have banned fidget spinners from lessons - here's why."
A number of teachers posted on the Mumsnet forum, complaining that the toys were ruining lessons.
One said: "I've had two children bring them in today 'because it helps them to concentrate' - no, it helps them to annoy their peers and stops everybody else from concentrating [sic].
"They are now in my desk drawer waiting for their parents to come and collect them."
Fidget spinners are marketed as tools for children with autism and ADHD.
One primary school teacher said they were included in the school's budget: "Specialists coming into the school recommend them for children and we'll buy them in for the children that are identified."
There is no supporting scientific evidence and at least one expert has debunked the claims.
Dr Mark Rapport, director of the Children's Learning Clinic at the University of Central Florida, said: "Using a spinner-like gadget is more likely to serve as a distraction than a benefit for individuals with ADHD."
However, the UK's National Autistic Society said there was anecdotal evidence that the spinners are beneficial.
Carol Povey, director of the society's Centre for Autism, said: "Having something that spins or twists can help to ground and balance them [autistic children in school]."