It has been discussed in numerous posts on threads on this topic. Here is one “for instance”
https://www.edweek.org/ew/articles/2018/03/09/this-weeks-nationwide-student-walkout-6-things.html
6.) How can schools accommodate students who don’t participate and/or don’t agree with the politics behind the walkouts?
Some districts where events are planned are also making it clear that students can opt out. In Allegany County, Md., students and staff members who don’t want to take part can spend time in school libraries engaged in alternative activities.
https://www.cnn.com/2018/03/11/us/national-school-walkout-march-14/index.html
Meanwhile, school officials with Chesterfield County Schools in Virginia have said their students can either hold a student-led memorial service for the victims of the Parkland shooting or hold a student-led assembly about school safety, CNN affiliate WTVR reported.
Well, as far as my local schools go it went pretty much as expected. Around 10% of the students left class (sunny day) and stood around for 17 minutes. There were no large signs or banners but might have been small ones. Pretty much a non-event - from a distance looked like they were outside for a fire drill.