That's the point -- it's not working with alcohol, why do you think it would work with marijuana?
Also, legalizing marijuana gives it legitimacy. It says that it's OK, and the kids will pick up on that (as they do with alcohol and cigarettes).
IMO, with legalization, usage among teens would increase as it did in Alaska:
"The consequences of legalization became evident when the Alaska Supreme Court ruled in 1975 that the state could not interfere with an adult’s possession of marijuana for personal consumption in the home. The court’s ruling became a green light for marijuana use. Although the ruling was limited to persons 19 and over, teens were among those increasingly using marijuana. According to a 1988 University of Alaska study, the state’s 12 to 17-year-olds used marijuana at more than twice the national average for their age group. Alaska’s residents voted in 1990 to recriminalize possession of marijuana, demonstrating their belief that increased use was too high a price to pay."
-- usdoj.gov
No proof of increase there. The following source suggests that it decreased, but not as fast as in the lower 48:
"Finally, in at least one of the United States, Alaska, decriminalization of marihuana, which took place in 1975, was deemed to be a failure. Notwithstanding a reduction in the daily teenage marihuana use nationwide of 75% since 1978, in Alaska it increased to twice the national average." - Juan R. Torruella, Chief Judge, First Circuit, Court of Appeals