Heather Hanbury, head teacher at Lady Eleanor Holles in west London, advised parents to ban phones, televisions and computers from their children's bedrooms.
"Students often claim they need their phone to wake them up in the morning," Mrs Hanbury told the BBC.
But she said mobile phones were distracting them from sleeping.
"Young people are regularly online, dealing with social media distracted by the idea of missing out if they're not online," she said.
All students at the Hampton school were given an alarm clock at an assembly marking World Mental Health Day.
The £20,000-a-year private girls school also carried out workshops for students on how "to rewire an anxious brain".
In a blog on the school's website, Mrs Hanbury wrote: "Without a proper amount of sleep nightly, it is very difficult to learn efficiently and effectively.
"Neurotoxins which build up during the day as we learn and experience things, can only be cleansed from our brains by sleep."
The NHS recommends children aged 12 and above get at least nine hours sleep a night, with those that don't more likely to be overweight or obese.
Persistent sleep-deprivation can leave also children overactive, seeking constant stimulation and unable to concentrate, it said.