Revision and snacking go hand in hand, found researchers
Junk food could be hampering the chances of exam-stressed youngsters' success, it is claimed.
Eight out of 10 youngsters told researchers for the School Food Trust they were likely to snack more and eat less healthily when revising.
A quarter of the 500 UK teenagers polled used caffeine-rich energy drinks, while 11% used caffeine pills.
Trust chairwoman Prue Leith said there was clear evidence that "smart eating" boosted brain power.
'Crash and burn'
She said: "It's often said you get out what you put in - our research shows that children are able to perform better in class when they have had a healthy school lunch rather than junk food.
"Children aren't stupid and they know that healthy food is better for them, and that a healthy breakfast and a balanced school lunch will give them more energy for their studies and help them concentrate more.
"Making that choice, and sticking with it, especially come exam time, could be the difference between success and failure."
However, some 42% said they had skipped meals to make time to revise, while nine in 10 regularly felt tired because of their school work.
The trust said that as a result, half of the youngsters it surveyed could only study for 30 minutes before losing concentration.
Yet nearly half agreed that eating a proper lunch helped them concentrate better in lessons.
And children tended to be aware of which foods would fuel their brains and aid concentration. These include fish, fruit and vegetables.
The trust recommends starting the day with a healthy wholegrain cereal such as muesli or porridge or wholegrain toast, swapping fizzy drinks for water and caffeine for herbal tea.
It also warns that energy boosting snacks like sweets and biscuits may give you an instant hit but will make you "crash and burn" soon after they release their energy.
Instead, it recommends, pupils chose healthy snacks and take regular breaks on their revision every two or three hours.