I wonder how many people walked out just to avoid class for 7 minutes.
It was 17, not 7. My school counts students truant if they miss more than 15 minutes of class. I don't think anyone did--you don't get any make-up time for work missed during a truancy. I don't think anyone was willing to get that sort of mark on their record in exchange for 17 minutes of standing around outside an office building chanting with signs. I think pretty much everyone was there for political purposes, or maybe in a few cases just from peer pressure (although 5x as many students remained in class... so...).
This is something I've heard from a lot of people, that we only left class to get out of our work. But that's frankly ridiculous. 17 minutes of protesting isn't very much fun. If it was an all-day skip where we went to laser-tag or something, that might make sense.
My school tried to screw us over in an illegal way, basically telling teachers to refuse to allow students who left class to finish their work once they got back. They changed that policy pretty fast (at least for my classes) after they got a not-so-friendly call from my parents (who are attorneys).