A British teaching union has branded caffeine and energy drinks as "legal highs" and warns of their negative impact on pupils' behaviour in schools.
The NASUWT teaching union is working with drug and alcohol charity Swanswell to examine the consumption of drinks such as Red Bull, Monster and Relentless.
Teachers have reported increasing concerns about the drinks' effect on behaviour, concentration and energy levels.
Some 13 per cent of teachers who responded to a NASUWT survey cited such drinks as a cause of poor pupil behaviour.
NASUWT general secretary Chris Keates said: "This is the first time we have seen a significant number of teachers beginning to raise this as a concern.
"These drinks are becoming increasingly popular among young people and are often seen as simply like any other soft drink, but many young people and their parents are not aware of the very high levels of stimulants that these drinks contain.
"They are readily available legal highs.
"Teachers are growing increasingly concerned that some young people are using these drinks to enable them to stay up into the early hours of the morning and then replace their lost energy by drinking two or three cans of these drinks on their way to school.
"Teachers are reporting that this affects concentration in class and hyperactivity is then followed by the inevitable crash later in the school day when the impact of these drinks wears off."
Swanswell says children should consume a maximum of 200mg of caffeine a day.
Its figures show that 500ml cans and bottles of popular energy drinks contain 144-160mg of caffeine.
CEO Debbie Bannigan said: "As a provider of drug and alcohol services for young people we're concerned about excessive energy drink consumption, not least because we're finding links to use of other substances, including alcohol, cannabis and Mephedrone".
British Soft Drinks Association director general Gavin Partington said BSDA supports the School Food Trust's rules on what drinks can be sold in schools, "but, of course, it is for teachers to decide what pupils are allowed to take into school".
