Because they are doing what you have failed to do:
High school principals made a robo call to parents this week telling them the event planned by CHC Enough, a student-led anti-gun violence organization, would be viewed as a school disruption and violation of the Student Code of Conduct.
"We fully support student voice, however it must be done in a way that is safe and doesn’t negatively impact education," the principals said in their call. "The April 20 walkout as currently promoted is a disruption to the school day, and a violation of the student code of conduct - which will be enforced."
This isn't just a local thing, the entire world is watching. It's the anniversary of the Columbine school shooting, which took place 19 years ago, and should have been a catalyst for changes to laws surrounding the purchase of deadly weapons. Yet we still have loosely-regulated gun shows and pretty much no control over the sales of "long guns." In a word, nothing happened. It's that failure of society and the government it relies upon that has brought this about, and punishing those students for what amounts to protecting their own lives would be shameful. And if you want to see the quintessence of hypocrisy, check this out: