Back to school is a tough time for children. As well as all the usual worries that come with starting a new school year, there are the added pressures that come from spending time online and using social media.

A new study by Internet Matters shows that half of all schoolchildren wouldn’t tell their parents if they’d been upset by something online – so just because they don’t say anything, that doesn’t mean nothing’s wrong.

If you’re worried something might be troubling your children online, here’s how to decode their digital lives.

[Read more: Staying safe on social media: Advice for parents]

Children are more reluctant to talk about online issues like cyberbullying than they are about those in the real world. In a survey of over 10,000 schoolchildren, 52% wouldn’t speak to their parents if they’d been upset by something online. In comparison, 91% would tell them if something upset them face-to-face.

It seems parents are often the last to know something’s wrong. In the study – commissioned by not-for-profit organisation Internet Matters – 60% said they were more likely to tell the police, 59% their friends and 50% another family member before telling their parents.

Bottling up these problems is causing children severe distress. Of the 10,028 six to 18-year-olds surveyed, 24% admitted that worrying about something that happened online stopped them sleeping.

To help parents understand these hidden digital anxieties, Internet Matters has made four videos showing how to decode the signals. Each focuses on a different topic: cyberbullying, sexting, inappropriate content and the pressures to be popular online.

Each shows how children’s dismissive, one-word answers can often hide their online torment.

[Read more: Online reputation: What is it, and how to keep you children safe online]

Dr Linda Papadopoulos, a child psychologist and body language expert, says parents need to get beyond their children’s one-word answers and find out what’s really going on.

“A new school year can be an intense time for children, full of change and full of excitement,” she says. “Beyond the day-to-day emotional weight of starting a new term with new friends and fresh challenges, they may also face extra pressure in their online world, which can make them anxious.

“They may feel under increased strain to have a persona online, keep up with old and new friends, or simply try to feel part of a group.

“As their dynamics change, it’s important that parents encourage their children to open up about their online activities and find out if their children are anxious about anything, as well as learning about some of the issues they might be facing including cyberbullying, sexting or seeing inappropriate content.”

You can find out more on these subjects from Internet Matters’ guide to starting the school year safe online.

You can also use BT’s Parental Controls to limit your children’s access to the internet at home. This can block content like social networking and media streaming, and lets you set homework time filters. Find out more here.

For further information about sexting and other online safety concerns visit Internet Matters' website - an online safety organisation offering a wealth of free, practical advice to parents and carers to help keep children safe.

[Read more: 12 tips to stop over-sharing online]