It was the shrug that got to me. My 12-year-old son had taken part in a class debate that afternoon. The subject was the second amendment. If the founding fathers were around today, what would they think about Americans and gun control? My son thought they’d take an extremely dim view of the whole situation and he had spent several hours preparing his argument, trying to avoid the words “moronic” and “completely insane”. I was at home, wondering how he was getting on, when the phone rang.

It was a recorded message from the headteacher. Her voice had that calm, absolutely-no-need-to-panic tone that immediately strikes terror in a parent’s heart. There had been a shooting a few miles from the school … the doors had been secured until the matter was resolved … someone was keeping an eye on all entrances … no direct threat … the measures were only precautionary.

By pick-up time, the incident was over. The shooters had been arrested far away, their crime not school-related. My son hadn’t even known the school had been put on alert. I asked him how the debate had gone. “Bit of an irony, you talking about gun violence while all that was happening,” I said. He nodded, thinking for a moment. Then it came. The shrug. He was ready to talk about something else.

When I moved to the US in the mid-90s, the subject of guns and gun control felt a bit abstract. Being British, I could coast along with a certain amount of emotional distance. It wasn’t my country.

Then I had kids.

We live in Massachusetts, a state with the third strictest gun laws in the nation. My kids don’t walk past shops selling guns, or see members of the public openly carrying firearms as they would if we lived in many other states. But it doesn’t take much to be reminded about gun violence.

I heard Sue Klebold being interviewed on the radio recently. She’s the mother of Dylan Klebold, one of the killers involved in the Columbine high school massacre in Colorado in 1999. Her memoir, A Mother’s Reckoning, came out this year. My eldest son was only a few weeks old when the shooting happened. I sat nursing him and watching the coverage, unable to take my eyes away. Along with all the fears of new parenthood came another. I live here, this could happen to my child.

Later, it was disconcerting to wonder if there was a gun in the house when I dropped my small children off for playdates. I didn’t think I knew anyone who owned a gun, but how would I know? All sorts of people might have one. Like the monosyllabic guy who collected his son from school in a pick-up truck. Or the woman who looked like Laura Bush and lived in a huge house with a US flag hanging from the porch. Probably neither did, I thought. Or both – only they kept the guns locked up, so it was OK …

Then there was the matter of toy guns. I felt more than a little uneasy to see my boys running around, squealing with delight as they shot each other, when hundreds of kids got shot by real guns every year. In the cosy liberal bubble where I live, many parents don’t allow toy guns.

“We don’t believe in them,” one couple told me, sipping coffee in my living room. Moments later, their five-old rushed into the room, his face alive with joy, waving a cowboy pistol he’d found at the bottom of my eldest son’s toy box. It was a while before it could be prised from his hands. He didn’t visit us again.

“We don’t have Nerf guns at our house,” another mother said rather pointedly, when she dropped her kid off for the afternoon one day. My house was full of the foam-bullet guns at the time, and of course the boys wanted to play with them, and of course her kid was the one who got accidentally shot in the eye. “I’m so sorry,” I said feebly, when she came to pick him up.She hustled her son away as he clutched his face melodramatically.

But along with the mortification of these moments, I couldn’t help feeling impatient. You don’t “believe” in toy guns? I thought. Really? How about not “believing” in real guns? How about that for an idea?

It wasn’t until the 2012 Newtown shooting in neighbouring Connecticut, that the whole thing really came home. When I picked my younger son up from school that day, nobody spoke. The secretary looked as if she had been crying. A few weeks later, my son’s lovely, friendly school had installed a new security system. You couldn’t just walk in to the place any longer. Now you had to press a buzzer and identify yourself. The kids were issued with photo ID cards that they needed to get into the building. If they forgot their card, they had to hang around until somebody opened the door. I felt desperately sad – and desperately relieved. What had happened at Newtown could happen anywhere.

It wasn’t just the security system that was new. There was also a new drill. The kids were taught what to do if an intruder entered the building. “It’s different from the fire drill,” my son tells me. “That’s just a loud noise. The other one is a voice that sounds a bit like Siri [on an iPhone].” The school is in lockdown. Make your way at once to the nearest classroom.

In the classrooms, the blinds are quickly drawn and the children line up against the wall so that they can’t be seen through the window in the door. Anyone looking in will hopefully assume that the room is empty.

The drills happen about once a year, always without warning.

“Is it scary?” I ask him.

“Only for a minute. But I know it’s probably just a drill. It’s really, really unlikely to happen for real.”

“Yes,” I agree. “Incredibly unlikely.”

But I can’t get over that word probably. I know someone with a gun won’t go to my son’s school. If they do, they won’t be able to get in. If they get in, the teachers and the kids will know how to keep safe. Probably.

• The Secret Life of Daisy Fitzjohn by Tania Unsworth is published by Orion Children’s, £6.99. To buy a copy for £4.99, go to bookshop.theguardian.com or call 0330 333 6846 Free UK p&p over £10, online orders only. Phone orders min. p&p of £1.99.