I'm all for a good conspiracy theory, but real tinfoil hat stuff like this only makes the gun rights community look bad. Unless someone produces actual evidence to the contrary, the simplest explanation is usually the correct one. Not actors, or imaginary highschoolers, or CIA plants. Don't be the guy who does stuff like telling everyone who'll listen that Michelle Obama is crossressing man and Barack is his gay lover.
What is entirely likely is various anti-gun and/or left-wing groups have stacks of gameplans for every possible shooting situation. At Parkland they found a bunch of trendy, diverse , outspoken kids so they pulled out the "Start a Youth Movement" binder and got everyone on board with promises of fame and fortune.