Researchers at the Institute of Psychiatry in London and the MRC Cognition and Brain Sciences Unit in Cambridge, found that teenagers' brains work less efficiently when they become dehydrated, making tasks such as problem solving far harder.
They claim that hot conditions, such as a poorly ventilated classrooms or exam halls, could lead to pupils becoming dehydrated enough to effect the neural activity in key parts of their brains which, as a result, have to work harder to achieve the same tasks.
Dr Matthew Kempton, who led the research at the Institute of Psychiatry's centre for neuroimaging, said: "In our experiment we looked at dehydration over a short period of time by asking participants to cycle while wearing impermeable plastic clothing.
"We used this as a 'model' of dehydration, but it is known that schoolchildren, especially in hot environments, suffer from moderate to severe dehydration.
"Our work suggests that dehydration negatively impacts brain functions underlying important cognitive processes in young people.
"These findings may be relevant to school policies regarding the availability of water as it has been shown that schoolchildren may suffer from moderate to severe dehydration, especially in hot environments. Many teachers do not appreciate the importance of children being adequately hydrated."
The researchers carried out brain scans on ten teenagers who were asked to perform 90 minutes of exercise to induce moderate dehydration before being given a series of problems solving and reaction time tests.
Although the children lost just 1.1 per cent of their body weight through dehydration during the tasks, the study showed that their brains had to work harder to carry out the same task when dehydrated.
The children's ability to perform the tasks did not change, but functional Magnetic Resonance Imaging scans revealed increased neural effort in key parts of their brains such as the middle frontal gyrus and the inferior parietal lobule – key areas in problem solving and learning.
The researchers, whose work is published in the scientific journal Human Brain Mapping and was funded by Water for Work and Home, also warn that long term dehydration or more severe dehydration could produce impairments in cognitive performance as the brain struggles to compensate.
Previous studies have shown that in adults, dehydration causes the brain to shrink in volume.
Earlier this year the British Nutrition Foundation issued new advice on staying hydrated after warning that dehydration could lead to a range of problems including headaches, fatigue and even depression.
The average Briton drinks just 200ml of water a day – less than one glass – while the Food Standards Agency recommends that people drink 1.2 litres, between six and eight glasses of liquid, to remain well hydrated.
Bridget Benelam, senior nutrition scientist at the BNF said: "It's not just hot weather that puts people at risk of dehydration. When it's cold, people may drink less. This may be due to a reduced thirst response or simply because we don't feel like chilled drinks in cold weather."