It was on a day like this, in weather like this, a decade or so ago, that I first learned the perils of social media.

It was a midweek night in January. Snow had been forecast to fall heavily the next day. So, in a seasonal ritual that many school and college leaders will recognise, I’d spent one of those long, lonely evenings obsessively tracking the Met Office forecasts, followed by a mix of fitful sleep and hourly glances through upstairs windows into the darkened world outside.

The question was simple: would I need to close the school next day?

I knew that whatever call I made, in that murky hinterland between night and day, once the working hours arrived the flak would fly - in letters to papers, local radio phone-ins, and, increasingly, through social media.

That day, the complaints started mid-morning - phone calls, emails, online posts. Some came from parents. Most, in truth, emanated from people who had little to do with our school but wanted to comment anyway on why we, amid the snowflakes, were being snowflakes.

Their anger centred in part on why I had chosen to close the school. But some also vented about something else. A member of staff, on a personal Facebook account, had written a message to friends that then gained wider attention: “Yippee. A snow day. I can get my marking done.”

That innocent celebration of unexpected recreation mixed with an opportunity for productivity triggered a toxic response. It was a tiny incident that made me much more aware of the blurring between our private and public lives, which is now the norm for teachers and for all of us.

What you post online stays online

Since that day, it’s an issue I’ve become highly attuned to. At staff meetings, I would say: “Don’t post anything online that you wouldn’t want to see on the front page of the Daily Mail.” I would warn them not to like or retweet content that contained any hint of inappropriate behaviour or bad language - including the routines of favourite comedians. If you retweet or like, I’d say, then you end up endorsing.

At interviews I would ask every candidate: “Is there anything online featuring you that you wouldn’t want me to see - photos, movies, being embarrassed, being embarrassing? Is there anything which could damage the reputation of this school?”

I’d go on endlessly about privacy settings and remind students that the version of themselves they post now will be indelible and inescapable in the future. Employers like me will Google you before we give you a job. The person you were may undermine the person you become.

It is just one aspect of a new landscape that is challenging enough for adults, but much more so for young people. So I welcome the children’s commissioner’s recent report Life in Likes, which examines the way children use social media in those vital transitional years from upper primary to lower secondary school and its effect on their wellbeing.

In it, Anne Longfield writes: “Receiving ‘likes’ from friends and peers was important to the children spoken to in this study. ‘Likes’ were seen as an affirmation and validation from peers that made children feel good about themselves and their lives.”

That’s why her call for mandatory digital literacy lessons shouldn’t be dismissed as another curriculum burden. Because this isn’t just straightforward mechanical knowledge about settings and preferences. This goes to the heart of what it is to be a human being.

In a life of likes, our inner identity becomes dependent on external validation. It means that my sense of personal wellbeing - even within the physical confines of my home - relies on what people out there say about me.

As Mary Aiken puts it in her fascinating but often chilling book The Cyber Effect: “We cannot gamble with the future development of children who will someday be adults who weren’t cared for and raised in the best way.”

So there’s work for us to do here - with parents - to make sure it remains safe to be a child. So that children are not, as Aiken puts it, hastily propelled from age 10 to age 18 because of pressures to conform to warped social expectations.

There’s work to do in our classrooms and assemblies, perhaps deploying older students to teach and mentor the younger ones to navigate their way through the internet, through adolescence, with increasing confidence and security.

And perhaps there’s a third responsibility for those of us working in schools - to exemplify what responsible online behaviour looks like. In my early Twitter days, I used to take delight in posting pictures of shop and restaurant signs where the apostrophe was in the wrong place - misspellings like “menu’s” and “tomato’s”. Using the hashtag #ApostrophePolice, my merry band of followers and I would take gleeful delight in celebrating the grammatical knowledge we had and that others clearly didn’t.

Then I read Jon Ronson’s So You’ve Been Publicly Shamed. In it, he described the devastating effect on people humiliated through social media. I realised that my silly grammatical posts were actually just a step away from Ronson’s warning about publicly sneering and rebuking.

So I waved goodbye to the Apostrophe Police.

In helping our young people to understand the protocols and conventions of social media, it’s about more than what and how we teach them. It’s also about exemplifying to them what principled, appropriate and ethical online behaviour looks like.

More than ever in a confusing world, children need to look to us adults to set the tone.

Geoff Barton is general secretary of the Association of School and College Leaders. He tweets @RealGeoffBarton

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