A couple things...

1. The event seems to have been poorly organized, nobody knew who was there or why. This includes both the organizers and the students.
2. This was definitely 100% put on by a student gun control group, with a small number of local pro gun control politicians arranged to speak during a portion. It seems that much of the official publicity about this event made it clear what it was all about, but that some texts being circulated did not.
3. Douglas County is a very red county (over 2.5:1 Republicans to Democrats) with very low esteem for Democrats (commies, socialists, what have you) very low gun control support. There is a very strong core of Republican kids in the school district, including the STEM school.

I live in Douglas County, and know families with kids that attend the STEM school. My own daughter would likely be attending - it's a great school - if not for transportation issues (no busing as it is a charter school). I personally believe this was a combination of genuine confusion and people being upset about the kids seemingly being excluded from the first portion. Several students did apparently speak, but they were part of the group that put the event on.

However, I think a lot of the actual anger was politically driven by kids from very red families that simply hate the idea of control and hate Democrats. In a very emotional situation it was probably not very hard for people to fall into line, especially if the instigators have any sort of social clout (which it seems likely they do).

Was it planned? I'm not convinced that it was, I think it was just a natural "fucking Democrat swine" response that a lot of people here knee-jerk towards. The hate that was simultaneously aimed at the media makes me think this even more.