Can’t imagine a lot of high schools in the US are gonna let my late 30s ass onto campus and start talking to students about why the military is evil.

This is true, but it's also not too difficult to reach those kids on social media, which a lot of military recruiters are also using. It might be possible to influence at least some high school students, who could then in turn influence their friends and classmates and push them against joining. Might also help to encourage political organizing among high school students themselves against recruitment efforts, wherever possible.