Do you want your children to do better in exams? Then take away their smartphones. (Sorry, kids!)

Schools that ban students from carrying phones see a clear improvement in their test scores, according to a study by the London School of Economics.

"We found the impact of banning phones for these students equivalent to an additional hour a week in school, or to increasing the school year by five days," researchers Richard Murphy and Louis-Philippe Beland said.

The authors looked at how phone policies at 91 schools in England have changed since 2001, and compared that data with results achieved in national exams taken at the age of 16. The study covered 130,000 pupils.

It found that following a ban on phone use, the schools' test scores improved by 6.4%. The impact on underachieving students was much more significant -- their average test scores rose by 14%.

"The results suggest that low-achieving students are more likely to be distracted by the presence of mobile phones, while high achievers can focus in the classroom regardless of the mobile phone policy," the economists said.

Related: He codes, he live-streams, he is 11.

Murphy and Beland said their study doesn't mean phones and other technology can't be used to boost learning.

"There are, however, potential drawbacks to new technologies," they said, citing the temptation to text, play games or chat on social media.

The use of mobile phones in schools is an explosive topic, with parents wanting to be able to reach their children and teachers complaining about disruptions.

New York City Mayor Bill de Blasio lifted a decade long ban on cell phones in schools in March, leaving it up to each school to set their own rules on phone use.

But Murphy and Beland said the decision may backfire.

"Schools could significantly reduce the education achievement gap by prohibiting mobile phone use in schools, and so by allowing phones in schools, New York may unintentionally increase the inequalities of outcomes."