Plus any students not favoring gun control will be afraid to say anything or to NOT march for fear of being ostracized by their peers.
Anyway, high school (and/or middle or elementary school) students don't know nearly enough about the multitude of factors involved to have a coherent opinion. Any demonstrations, no matter what they say, aren't really about "gun control," they're really about "we don't want to get shot at school" control. But all they're seeing in the media is that gun control will make them safe, so that's the buzzword for the day.