[…] the percentage of trans people in the US between 1999 and now would’ve been on an upward trajectory […]

A larger fraction of the population in most western countries are certainly openly trans now, than were in 1999. But I don’t know about all trans people, I think the fraction has stayed the same, but coming out has become easier (I’m not saying that it’s easy, I’m saying that I believe that it’s easier than in 1999)

But let’s assume that you’re right, and fewer people used to be trans compared to the population size. That would mean fewer expected school shootings by trans people. Which could explain how the expected interval in my calculations is 2-7 shooters while we’ve only seen one. Once again confirming that trans people are just people.