I knew I’d been mean at school. But when an old friend told me I was such a horrible bully I’d left one girl needing therapy, I felt sick…

I was recently reminiscing with some old school friends over drinks, talking about the teachers we had crushes on and rolling our eyes at the extreme lengths we went to shorten our skirts.

Then a few people from our group told moving stories about the enormous pressure they felt at school, one of the best girls’ schools in the country.

“It wasn’t all bad,” I said lovingly. “Well, you could say that,” my friend Jo shot back. “You were a bully.”

The word felt shocking and strange. Me? A bully? I laughed, thinking she was joking. We were still friends, after all, all these years later. But something in her expression told me she was serious.

When I think of a school bully, I picture a muscular thug waving his fist and stealing someone’s lunch money. I never did anything like that. But the more Jo and I talked about it, the more I had to accept that I had been involved in causing pain, sadness, and humiliation to my classmates.

I’m ashamed to admit it, but at school I was a mean girl.

The 2004 film came out just as I was fresh out of high school, but I really liked the way it depicted the cruel hierarchies of teenage girls.

From the age of twelve I was part of a girl gang of fifteen people known as ‘La Clique’. If we didn’t like someone, we could become aggressive.

When I read Shona Sibary’s piece in the Mail last week about the lasting effects of bullying she suffered at the hands of a fellow pupil, I was hit with a fresh wave of guilt. I am on the other side, but the shame of how I behaved still makes me sick to my stomach to this day.

Back then I didn’t see myself as a Queen Bee type. I was more filled with self-loathing and constantly worried that I wasn’t pretty enough, smart enough or popular enough with boys.

It was probably these insecurities that made me do what I did, desperate to fit in and not be ostracized by the popular girls. I was too afraid to speak up when we did something I knew was wrong, for fear of losing my status or having the pack turn on me.

Further evidence of my Mean Girl status was confirmed when I pulled out my high school yearbook. Every photo showed me and my friends arm in arm, often in matching outfits, with captions like, “You can only be friends with us if you’re thin and pretty.” I like to think we were being tongue in cheek, but deep down there was truth to it. We were intimidating, and we enjoyed it.

As an adult, it was painful to admit that I wasn’t a very nice person at school. When Jo first confronted me, I reassured myself that if I was a bully, it was only verbal. But in retrospect, that’s not entirely true.

One of my friends Sarah, who was very well developed at a young age, reminded me that we created an elaborate ritual, complete with songs and dance steps, where we pressed heavy dictionaries into her chest to flatten her breasts. We pushed balloons into our itchy blue V-neck sweaters to imitate her.

From the age of twelve I was part of a fifteen-man girl gang known as ‘La Clique’, and we could be pretty mean

The 2004 film Mean Girls came out just as I was getting out of high school, but its descriptions of the cruel hierarchies of teenage girls really spoke to me

Of course we were just jealous that we didn’t have breasts yet, but afterwards she told us how painful it was (both physically and emotionally).

“Pranks” like this often escalated into physical intimidation. We frequently shoved “Little Maria,” the smallest member of our group, into a locker.

We pretended to attack each other with hockey sticks, actually did it once, drawing blood and shattering a few orthodontic-aligned teeth. Someone cried at least once a day. We didn’t have the term “body shaming” back then, but we were masters at it.

It was dog-eat-dog, and while I participated in the meanness, I was also sometimes a victim. I remember a girl in English class telling me to hide under the table, and when I obediently did so and asked what was wrong, she said, “I’m hiding things I don’t like.”

I was also painfully thin as a teenager, and other members of my group would force me to eat packets of butter in the lunchroom while they chanted and laughed. Sharp remarks, cruel comments and name-calling were all part of the fun.

Except it wasn’t fun. It must have been deeply traumatic and disturbing for those involved.

I know that at least one member of our group has had therapy as an adult because of her experiences with us. I couldn’t believe it when she told me; I apologized with tears in my eyes, but I knew the damage had already been done.

If I were to analyze what made me act this way, I think it was probably the heady combination of wild hormones and intense boredom. We were smart and ambitious, but had little opportunity to express ourselves other than all that game-playing and backstabbing.

At school it was a battle between the two, and although I took part in the meanness myself, I was also a victim at times

For me, my chaotic home life was undoubtedly a factor as well. After my parents’ tumultuous divorce, my mother left me when I was 15. Exercising power over my classmates was one of the few ways I could feel in control.

I’ve had my fair share of therapy myself, and I hope it’s made me a kinder, more empathetic person. I’m grateful I went to school in the pre-Instagram era, because I fear the insidious cyberbullies I would have gotten into if we’d lived online. And how the record of that would live forever, for all to see.

I don’t remember teachers ever interfering or trying to discipline us. It all felt rather lawless, like Lord of the Flies armed with Impulse body spray.

Although I like to think that I will never fall back into my old behavior, I sometimes catch myself relapsing.

One glossy magazine I worked at was like being back in the classroom. Everyone in the office wanted to be at the ‘Top Table’, where a group of powerful women mocked those below them in the pecking order. I had to resist the urge to sit down when they disparaged a junior staff member’s ideas, or didn’t invite certain people to after-office drinks.

I’m not proud of it, but I can’t deny that I still enjoy gossiping and that it’s a good way to quickly build bonds with others.

I’m still friends with most of La Clique — a few of us even went on vacation together recently. When we talk about our school days and the times when the jokes went too far, everyone is embarrassed. We all agree that it was really weird. I’m sure that, like me, they wouldn’t see themselves as the bully.

Nowadays, when we spend time together, there is much less bickering. But our WhatsApp group still shows the same old gossip antics, where supposedly supportive messages are in reality anything but that.

“She’s so brave to get a divorce, I would never do that to my children,” was a recent example. When I was younger, I would join in, but now I don’t. When I do, I’m left with an emotional hangover that takes days to go away.

Sometimes I look at my daughter and wonder if this toxic behavior in female friendships is innate or learned.

She’s only three, but I can already hear her and her friends telling a new girl who seems waddling, “You can’t play with us,” or saying out loud that someone “isn’t my best friend.” I would never send her to an all-girls school, and I hope she finds her teenage years to be a calmer, more compassionate one than mine.

But something tells me that the brutal dynamics of teenage girls will never change. Emotional manipulation, gossip and isolation seem to be the order of the day. I see them on the bus sometimes and see subtle signs of the full-fledged war they are engaged in.

It could be something as simple as body language that completely leaves one group member out, or a comment that makes everyone burst out laughing – everyone except one girl.

At least I can tell my daughter that the Mean Girl years get better. And the person you become to survive them will be better, too.

Names have been changed.