I was 6 years old when 12 students and one teacher were murdered at a high school in Columbine, Colorado. I don't remember much about the immediate aftermath of the mass shooting — how it was talked about on the news, what President Bill Clinton said on TV, or the many stories about survivors that would follow. What I do remember is that my mom picked me up from elementary school that day, and that I could tell she was scared. With the abstract understanding of a 6-year-old, I knew she was scared for me.

As I grew up, that fear, she has since told me, faded "for some reason or denial" — and for me, it blessedly never took root. It is only now that I can look back and realize that I was among the last groups of Americans to go through high school — I graduated in 2011 — and not have "school shooting" as a fear gnawing away somewhere in the back of my mind.

This week, watching teenagers run as fast as they could from their high school, waved on by men made anonymous by their SWAT gear and weapons, this realization hit me hard. A mother said on CNN that her daughter had practiced lockdown drills at school. A father told a local affiliate that he had demanded that his daughter, who was sheltering in place, turn off her phone so it wouldn't buzz or vibrate and alert the shooter to where she was hiding. As my colleague Matthew Walther chillingly observed, the parents and students in Parkland, Florida, "understood that what was happening was something called a 'school shooting,' an event with known conditions and procedures and conventions, something that has happened 230 times in this country since 2013." They knew — with the same robotic instinct with which one would stop, drop, and roll — exactly what to do.

I attended a high school in suburban Seattle that was half the size of Marjory Stoneman Douglas High School, but in many ways they are interchangeable. Both are located in comfortable and safe suburban neighborhoods. (Much has been made about Parkland being named the safest city in Florida last year.) Also like Parkland, my high school had open air hallways between classrooms (although unlike Parkland, in the suburbs of Seattle this design seemed whimsically optimistic in regards to the weather). It was only in the aftermath of shootings at similarly designed campuses that this feature seemed to be a morbid design flaw: Anyone can walk up to the school and do anything. Suspected Parkland gunman Nikolas Cruz seems to have taken advantage of the feature: "He began his shooting rampage outside Marjory Stoneman Douglas High School in this suburban neighborhood shortly before dismissal time around 2:40 p.m.," when he knew students would be pouring out of classrooms and into the open, The New York Times reports.

I'm sure all too many Americans have imagined what they would do during a mass shooting. I've realized, with a perverse fascination, that in large gatherings I now unconsciously scope out my exits. At a recent concert in a questionably-fire-code-safe building in Brooklyn, I thought about how easy it would be for a maniac to kill dozens in the crowd, and if I might be stupid for making myself such an easy target. In movie theaters, when the story slows, my mind drifts to Aurora.

I imagine the worst so that in some complicated, not-fully-understood way, I might be prepared when it happens.

That is what makes me wonder with dread just how many students at Marjory Stoneman Douglas had thought about exactly the scenario that unfolded Wednesday. It makes me wonder, despite how safe a neighborhood might be, how many American high school students could hear a Code Red over the intercom and after realizing it wasn't a joke, think impossibly: This is happening.

That would never have occurred to me in high school. That's the difference between my generation and teenagers today, even though we're only separated by a handful of years. Due to a combination of privileged innocence, trust in the world, and above all else, timing, I never felt unsafe in high school. My concerns were what they should have been: grades, if so-and-so liked me, where I might get into college, and on the more paranoid end, if there might be an earthquake. It would be grossly ignorant to suggest that every student in America was fortunate enough to feel safe in their school. But a mass shooting is an especially new and unique fear. Most of all — although still a terrific unlikelihood — it is a possibility.

There is no going back. No amount of gun reform can erase what has happened or the lives that have senselessly been lost (although that doesn't mean every effort should not be taken to stop it from ever happening again). Looking at Marjory Stoneman Douglas, it is horrifyingly evident that at some point, without us realizing or doing anything about it, American childhood changed forever.

This isn't just because of Newtown and Umpqua and Marjory Stoneman Douglas. It is because this is now an everyday reality: eight school shootings have somehow already happened this year.

I will never be an American teenager again. But one day, when I become a parent, I will be dropping my own children off at school. And when I do, I will have something my mother never did: a fear, deep inside me, that it could happen here too.