Prince William has made a surprise appearance at a London school on Thursday, offering sober advice for young women about self-image in the internet era.

The Duke of Cambridge addressed Burlington Danes Academy students at a secret midday assembly on the issue of cyberbullying, joining students Samara Hackett-Valton and Sophie Crowder, both 15, on stage to talk about the importance of mental health.

“I worry for you girls,” Prince William said in a candid admission.

“The touched-up pictures are not real.

“Don’t try to recreate them or think that’s what you’ve got to aim for. There’s a lot of fakeness online so don’t worry about that.”

He noted the juggling-act of stress young teenagers carry in modern society, saying: “That’s a lot. You must be aware of that, it’s a lot of pressure. There’s so many things going on. You’re going to be bamboozled.”

Prince William urged the students to put down their devices to promote mental health, telling the 250-odd students to use technology sparingly.

“Your speaker for this morning’s assembly is … err Prince William”. Duke of Cambridge & @professorgreen surprise pupils at @ArkBDA who are talking about cyber-bullying #StopSpeakSupport pic.twitter.com/NITrRpCGCE

— Chris Ship (@chrisshipitv) February 8, 2018

“Don’t spend all day online. Seriously, don’t,” he said.

“For your mental health, get outside, come away from the screen. By all means, be on a screen but don’t be on it all day because it will only bring you into another world. It’s important that you balance the time.”

He made a point of how often feelings can be hurt via text messages and social media, saying: “Unless you punctuate it correctly — I’m not the best at punctuation and I’m not the grammar police, either — you can read it in 100 different ways.”

He also counselled boys on the importance of emotional intelligence and learning to share your feelings.

“It’s really important for boys. We’re not very good at talking about our emotions and how we feel,” Prince William said.

“Girls have got a little bit better, and boys, we’ve really got to work hard on being able to talk to friends, family, and trusted people about how we feel.”