In 1971, five high school students in San Rafael, California, used the term “4:20” in connection with a plan to search for an abandoned cannabis crop, based on a treasure map made by the grower. Calling themselves the Waldos,because their typical hang-out spot “was a wall outside the school”, the five students — Steve Capper, Dave Reddix, Jeffrey Noel, Larry Schwartz, and Mark Gravich designated the Louis Pasteur statue on the grounds of San Rafael High School as their meeting place, and 4:20 pm as their meeting time. The Waldos referred to this plan with the phrase “4:20 Louis”. After several failed attempts to find the crop, the group eventually shortened their phrase to “4:20”, which ultimately evolved into a code-word the teens used to refer to consuming cannabis.
I don't smoke the stuff, but I remember wondering many years ago about the 420 reference, as well. I had basically understood that it was a reference to 4:20 PM, guessing that it was when the average person used to get home. After looking at it again today, I see there was much more to the story.
We had a sizable number of kids that hung around a wall, smoking cigarettes a deed, in my high school. They were called wallrats.