There are several school walkouts coming up in Colorado and nationwide that I plan to participate in, most likely without my parent's permission/knowledge. I am 16. Is this legal?
One of the relevant cases is Tinker vs. Des Moines.
In it, students wore black armbands in protest, and were suspended from school unless they agreed to remove the armbands. Because their protest was non-disruptive, the school's position was found to be a violation of their civil rights. Generally, the school cannot interfere with students' rights to free speech and civil liberties. That does not mean that students are free from discipline however.
If students walk out, whether in protest of a great injustice, or just to get lunch, or go to a concert, the school can discipline the student for walking out.
What the school cannot do, however, is treat the reason for the walkout differently. They cannot give minor detention for one reason (say, a rock concert), and a full suspension for politically motivated walk-out.
If they are dispensing harsher punishments based on the reason for the walkout, that is crossing a big line.
More detailed discussion can be found at http://time.com/5171160/gun-control-student-protest-history/