Sure, make it legal. That way people can openly and legally ruin their lives and get blitzed everyday. It may not be as harmful as alcohol or whatever, but the people who sit on their ass all day and get high are fucking themselves up.
As you said, it's under the responsibility of the man. If you smoke before a school test, you'll fail the grade; if you smoke before going to work, you will likely make a mistake and you'll get fired or - worse - you'll be convicted and put in jail.
In the countries where weed is legal (e.g. netherlands) the number of people that abuses of drugs is lesser than in other countries and the people tends to be more civil than in other countries; furthermore if you go in a coffee shop in amsterdam you will likely find more french, german or italian people than locals.
Few years ago in italy weed had been likened to heroine, cocaine and other heavy stuff. This resulted in an epic fail, since the addicts to heavy drugs are significantly increased and the price of heroine and cocaine dropped (10€ for a single dose of heroine); as this was enough we had the side effect of jails filled with people that will be imprisoned for 6-20 years and a fine from 26000 to 260000 € because they were busted whit few grams of weed

Is that reasonable?