actually I read up on it a bit, and Hitler's birthday really had nothing to do with the Columbine shooting. The original date was to be April 19 because that was the date of the Waco siege when Federal agents and Texas marshals stormed the Branch Davidian compound - - but the dealer who was supposed to supply the ammunition to the Columbine shooters was not able to deliver it in time so it was delayed a day.
At any rate, if students (and teachers or whomever) do walk out of school, I just don't think it should be on April 20 since many people already associated that with a day to get stoned. And generally, while I understand the motivation behind it, I don't think a school walkout for more than a day is practical. Even for one day, it's problematic. What ages are doing this? Officially perhaps it would be high school students, but you'll certainly get middle school and younger students who want to participate. And many (most) who "participate" probably will do it just because it's a day off of school.