Children as young as 11 are being treated for abusing the anxiety drug Xanax, the BBC has found.
Drugs charity Addaction said it was also aware of 13-year-olds "dealing" the tranquiliser on school premises.
The BBC has seen a number of letters from head teachers to parents raising concerns over increasing abuse of prescription medications.
One teacher said he feared pupils had made an assumption that taking Xanax was safer than using illegal drugs.
Xanax, the brand name for the drug Alprazolam, is used to treat anxiety and panic attacks.
It is widely prescribed in the US and can be obtained on private prescription in the UK. But among some teenagers and young adults in Britain it is increasingly being abused.
To gain an idea of the scale of Xanax abuse, the BBC sent a freedom of information request to six ambulance services about its call-outs to Xanax ingestion incidents where the patient was aged under 18.
The North East Ambulance Service was the service that provided the most comprehensive details about when Xanax had been ingested – other services recorded all incidents involving the family name of the drug, benzodiazepine – and it said that in 2017 it had attended 240 call-outs for Xanax abuse by children aged between 11 and 18.
Two of the cases last year had been for 11-year-old and there had also been cases recorded in 2014 and 2016 for children of that age.
'Risks not understood'
Nick Lind, deputy head of Redland Green School in Bristol, said that, over the last 18 months, he had written several letters to parents about Xanax abuse.
He said he was concerned that the abuse of prescription medication had become a trend, and that pupils believed it was "a safe alternative to illegal drugs", obtaining supplies mainly from the dark web.
"Students are finding out that that's the drug that's available, they've seen other people using it. Sometimes it's mentioned, because it is an anti-depressant and an anti-anxiety drug.
"Because it's seen as a prescription drug and therefore not seen necessarily by some people as dangerous because it is prescription, they don't understand the risks. They think it's a safe way of getting into drugs."
And he said that he had heard various reason why pupils were taking the drug as a form of self-medication. "They know the background of Xanax is as an anti-depressant, and they think they can use it when they're building up for exam stress or through difficult times in their lives."
Kerry Robinson, a paediatrician at London's Whittington Hospital, said she had treated children aged under 16 for Xanax abuse.
"The cases that I've seen it's not a recreational party drug," she said. "Young people have taken it during the school day, then stumbled into their next lesson and someone has picked up that things are not right."
She said symptoms included slurred speech, unsteadiness, and looking drunk, but without the lack of inhibition associated with alcohol.
Addaction said it was "concerned" by the number of teenagers it was helping that had addiction problems related to the drug.
Neil Coles, from Addaction, said that he and his colleagues were providing advice and support to a number of children as young as 13 caught using and "dealing" Xanax.
He said: "I don't think a lot of young people realise what Xanax is. They don't realise that it's a benzodiazepine.
"They don't realise that your tolerance builds to it very, very fast. They don't realise that if you mix it with alcohol it's problematic.
"So we're seeing a lot of young people who don't realise the dosage, they're taking far too much, collapsing on school premises, and therefore having to be taken to hospitals."
Mr Coles said teenagers were also buying the raw ingredients from the dark web to produce their own version of the drug.
That means many other teenagers fail to realise that the Xanax they are buying is not of pharmacy grade and is instead counterfeit, he added.
One 17-year-old, who wished to remain anonymous, said he started taking Xanax at 14 to calm his anxiety. He said he had mixed the drug with alcohol, and ended up in hospital "a couple of times".
He said: "People need to be careful, because you can take one bar and be feeling absolutely fine, and then they can take another one and then two days later wake up in a police cell or hospital with absolutely no memory of what happened."
A BBC Three investigation into Xanax found that, last year, Border Force officers seized more 50kg of the powder form of the drug – enough to make 25 million typical-sized pills containing 2mg each.
A Home Office statement said firm action was being taken "to prevent the harms caused by drugs".
It added: "Law enforcement agencies continue to work with internet providers to shut down UK-based websites found to be selling these drugs illegally."