Now, though, there is an air of … money. There’s no other way of putting it.

The silhouette of the tall building casting its long shadows across the croquet lawns and tennis pitches is the same, but the stark, cheap white paint has been changed to a deep expensive cream, subtly softening the edges, an effect enhanced by the flowers that have been placed in window boxes, and the creepers that have been planted at the corners of the building and are beginning to twine across the facade.

The lawns are lusher and greener, and as we make our way across them there is an almost inaudible ‘click’ and small spigots rise from the grass and begin spraying a fine mist of water, a luxury unimaginable when we were there. Outbuildings and covered walkways have sprung up, so that girls no longer need to scurry from lesson to lesson in the driving rain. And as we pass the all-weather tennis courts, I see they have been updated from their unforgiving knee-skinning tarmac to a kind of rubbery green sponge.

What hasn’t changed are the four towers still standing sentinel, one at each corner of the main block, the black skein of the fire escapes still twining up them like post-industrial ivy.

I wonder if the tower windows still open wide enough to admit a slim fifteen-year-old, and whether the girls break out now like they did then … Somehow I doubt it.

It is half-term of course, and the place is strangely silent … or almost silent. As we walk across the playing fields, cars sweep up the drive, and I hear faint voices coming from the front of the building.

For a minute my ears prick, and I think parents! with much the same sense of danger as a rabbit might think hawks! But then I realise – these aren’t parents, they’re girls. Old girls. Us.

Only not us. Because, somehow, it was always us and them. That’s the trouble with having a ‘click’ as Mary Wren might call it. When you define yourself by walls, who’s in, who’s out. The people on the other side of the wall become not just them, but them. The outsiders. The opposition. The enemy.

It’s something that I didn’t understand, in those early days at Salten House. I was so grateful to have found friends, so happy to have found my own niche, that I didn’t understand that every time I sided with Kate and Thea and Fatima, I was siding against the others. And that soon they might side against me.

A wall, after all, isn’t just about keeping others out. It can also be for trapping people inside.

‘Oh. My. God.’ The voice floats across the evening air, and we turn, sharply, all four of us as one, towards the sound.

A woman is approaching us, her heels crunching and teetering on the gravel.

‘Thea? Thea West? And – oh my God, you must be Isa Wilde, is that right?’

For a minute I go blank, and can’t place her name, and then it comes to me. Jess Hamilton. Captain of Hockey in the fifth form, and widely tipped for head girl in the sixth. Did she make it? I wonder. But before I can open my mouth to say hello, she’s barrelling on.

‘Fatima! I nearly didn’t recognise you with that scarf! And Kate, too! I can’t believe you’re here!’

‘Well …’ Thea raises an eyebrow and waves a hand slightly deprecatingly around the group. ‘Believe it now. Is it that unlikely that we made it this far in life? I know I had a live fast, die young poster on the wall of the dorm, but you weren’t supposed to take it literally.’

‘No!’ Jess gives a shrill laugh, and shoves Thea’s shoulder playfully. ‘You know it’s not that. It’s just …’ She falters for a second, and we all know what she’s really thinking, but she recovers and continues, ‘It’s just that, well, that you’ve never shown up to any of these things, any of you, even Kate, and she only lives five minutes down the road. We’d quite given up hope!’

‘How nice to know we’ve been missed,’ Thea says, with a little twisted smile. There is a moment’s awkward silence, and then Kate begins to walk.

‘So, gosh,’ Jess says, falling into step with us as we make our way round the corner to the main entrance. ‘What are you all up to? Kate, I know, of course. No surprise that she’s become an artist. What about you, Isa, let me guess – something to do with education?’

‘Nope,’ I say, forcing a smile. ‘Civil Service, actually. Unless you count trying to give ministers a crash course in law. How about you?’

‘Oh, I’m very lucky. Alex – that’s my husband – did awfully well out of the dot.com boom, he got in and out just at the right time. So we’re full-time parents to Alexa and Joe.’

Thea’s eyebrows nearly disappear into her fringe.

‘Do you have kids?’ Jess asks, and I nearly don’t reply, before I realise her question is directed at me, and I nod hastily.

‘Oh, yes. One little girl – Freya. She’s nearly six months.’

‘Home with the nanny?’

‘Nope.’ I manage another smile. ‘We don’t have a nanny in fact. She’s at Kate’s, with a babysitter.’

‘And how about you, Fatima?’ Jess continues. ‘I must say, I didn’t know you’d become …’ she nods at the scarf, ‘you know. A Muslim.’ She sort of mouths the last word, like someone not wanting to articulate something slightly taboo.

Fatima’s smile is even thinner than mine feels, but she doesn’t let it slip.

‘I was always a Muslim,’ she says evenly. ‘I just wasn’t very observant at school.’

‘And so … what … you know, what changed your mind?’

Fatima shrugs.

‘Kids. Time. Adulthood. Who knows?’ I can tell she doesn’t want to talk about it, or not to Jess, not now.

‘So you’re married?’ Jess says.

Fatima nods. ‘To another doctor. I know, right? What a cliché! Two lovely kids, a boy and a girl. They’re at home with Ali. What about yours?’

‘Same as you – a girl and a boy – Alexa, she’s nearly five – can’t believe where the time goes! – and Joe, who’s two – they’re at home with the au pair. Alex and I took a couple of sneaky nights off for a long weekend. Gotta have some couple time, right?’

Fatima and I exchange a fleeting look. I’m not sure what to reply to this – Owen and I have not had any ‘couple time’ since Freya was born – but we’re saved by the appearance of a tall blonde woman on the path up ahead, who does a mock feint, hand to her heart and then says,

‘Not Jess Hamilton? No! You can’t be, you don’t look old enough!’ ‘The very same,’ Jess says, with a little bow, and then she waves a hand at the four of us. ‘And you probably remember …’

There is a sudden silence, as I see the woman registering our faces, recalling names, and her expression changes, the polite society smile falling away. She does remember. She remembers exactly.

‘Of course,’ she says, and there is a kind of cold reserve in her voice that makes my heart sink. Then she turns back to Jess, linking her arm with hers, turning her back on the rest of us. ‘Jess, darling, you simply must meet my husband,’ she says conspiratorially.

And just like that Jess is swept up, borne away, and it’s just the four of us again. Alone. Together.

But not for long. For as we round the corner to the drive, the double doors are open, light streaming out into the evening air, and people streaming in.

I feel someone reaching for my hand, and when I look, it’s Kate, her fingers twining into mine for support, her grip painfully hard.

‘Are you OK?’ I whisper, and she gives a single, sharp nod, but I’m not sure if she’s trying to convince me, or herself.

‘Armour on?’ Thea says, nodding at the shoes I am holding in my free hand, and I realise, I’m still wearing my walking sandals, covered with dust. I kick them off, balancing against Kate’s shoulders, and slip my feet into the high heels. Fatima is doing the same, supporting herself against Thea’s arm. Kate’s dress flutters in the wind, like a flag – like a distress signal, I think suddenly, the image popping into my mind uninvited, and I push it away.

We glance at each other, and in each other’s eyes we read the same feelings – trepidation, nervous excitement, fear.