On Daily Rotten’s home page (no gore on this link) there is the following blurb at the top of the list of Today in Rotten History:
** Apr 20**
It's 4/20. If your kid cuts
school today, he just might
have a chemical dependency.
That's all we're gonna say.
Anybody have any clue what the heck this means?
4:20 is a marijuana reference - it’s the time to go get high (thus the “4:20 Club” and so on). I kinda doubt that most pot smokers wait until 4:20, none of my friends ever did.
Anyhow April 20th is 4/20 so there you go.
It’s interesting they would point that out; many high-school age stoners really do celebrate 4/20, and it’s something parents might want to know about. Anecdotally, I can’t recall it being a major thing in my high school, although marijuana certainly was widespread. Skipping school frequently is a problem in and of itself (and often related to drugs), and unusual activity on April 20th might well be a sign to parents that a child smokes marijuana often, if there are no other signs. Calling it a chemical dependency is not entirely accurate, since marijuana is not physically addictive, but it can become something of a habit. In particular, it can be a problem for high-school students if they skip classes and don’t do homework because of it; poor marks in high school means difficulty with college (or no college), and that may mean economic difficulty in later life.
The origin of the 4:20 thing is obscure; there are many different stories, and it’s difficult to determine which is true. At any rate, April 20th is Hitler’s birthday and the anniversary of the Columbine massacre, and a number of other bad things happened on April 20th. Granted, good and bad things happened in history on every day, but Hitler’s birthday is a pretty bad thing, and IIRC it was a major celebration for the Nazis.
If your child stays late after school on 3/14 (π day, since π is 3.14159…) or 10/24 (mole day, from Avogadro’s number, 6.02 x 10[sup]24[/sup], which defines the number of molecules in a sample of that substance with a mass equal to its atomic or molecular weight), they may be headed for a career in science. In my opinion, this may result in far greater hardship than the abuse of any mere drug (well, not really); it’s not too late to teach them the importance of money and having a social life rather than spending all of college studying and writing papers. =)
My stepson is a pothead. He has 420 written all over his stuff. It’s an inside joke with stoners, apparently. That is, if they can remember it.
The joke has been around for decades, so it appears they can. 
I don’t know what kind of a yutz would skip school just to get high when they can do it other days without the risk, but maybe it happens.