Here is the #MarchForOurLives permit on the NPS site (if it's real). It was submitted 2/20/18 by event organizer Deena Katz - "Hollywood producer and casting director." Although not submitted prior to the shooting, six days after seems a bit shady too. When you google it, the google date is 24 Mar.
https:// www.nps.gov/aboutus/foia/upload/March-For-Our-Lives-application_Redacted.pdf