If the targets were truly random and not based upon objective criteria then one would expect to see mass shootings happening almost anywhere and not heavily focused on schools and such. You coming up with one shooting that didn't follow that pattern isn't really a strong critique of the offered hypothesis that would-be shooters actively seek out places where they expect no armed resistance (such as a school). Another theory would be that would-be shooters prioritize people they know and/or think somehow "wronged" them so a school is likely if it's a young shooter. Potential problems with that theory is that older shooters don't seem to prioritize their workplace as much as younger shooters do.
So what's stopping them from targeting clubs, malls, or churches now? Once in a while you see a mass shooting at one of those places but it's far, far less prevalent than schools.
The targets are not random. The targets are strategic.
Shooters always want to send some sort of message. They pick targets to send their message.
1) Schools are extremely soft targets. I would say most malls are better defended than most schools in terms of security (guards, off duty police, concealed carry citizens, etc). In addition, in malls targets are way more spread out as compared to schools where a bell rings and 1000 people are in narrow hallways.
2) Schools are places where people have a lot of "perceived enemies" (students, teachers, administrators, etc)
3) Killing young people in
the place where society trusts the most that they will be safe imparts the most pain on society. It's not hard to understand this.