These kids are being terrified by their teachers. They feel powerless to do anything about it and they are a group that have been taught that demonstrations make things happen.
Give them something else to do. For example (I dont actually know anything about the group behind this movement but it seems like a good idea on the surface), there is a WalkoutUP movement. This is where kids run a campaign to include the kid who is always marginalized and befriend them instead.
Maybe this doesnt really do much but, it relieves that feeling of powerlessness and gives them that movement feeling which then takes the wind out of the antigun walkouts sails.
I like your idea of offering a deflection that appeals across the usual lines, something the inmates and guards will both embrace.
As for helping the marginalized kids, some of them are dangerous, and marginalized for that reason. My son was "stabbed" (with a pencil) while on the bus - no action taken. The kids will tell you who the next shooter is going to be. Dysfunctional homes. Schools can't fix it.