Of course, it couldn't have been because some adult instigators had advance knowledge that it was going to happen. (sarc. off)
From what I’ve been able to determine, the permitting process often takes months. They may have already scheduled an anti-gun march several months ago, and then took advantage of the shooting at Parkland school. As it was, the shooting occurred in the outer bands of the time window of the pre-scheduled march. To keep the march salient, they held impromptu “school walkouts” about 10 days before the march, just to keep the momentum for the “March for Our Lives” relevant.
Let’s up the ante a bit though. I do recall seeing some postings on the web over a year ago that the “Women’s March” of January 2017 was actually scheduled in August of 2016 and permitted sometime before the November 2016 election, with the expectation that Hilary would win. It was supposed to be a celebration and coronation of the US’s first woman president the day after Hilary would be helped up and down the steps on the inauguration platform to give her speech. Instead, history occurred and we Americans elected Trump into office instead. The already pre-scheduled march went on anyway, but was repurposed as a protest against Trump. I think a lot of the documentation about the original permitting of the “Women’s March” has been scrubbed from the web, however.
The left never lets an opportunity go to waste.