My view, for what it's worth, is that permanently excluding a first-time offender (by which I obviously mean "first-time caught"

for possession is entirely counterproductive. It's saying "make one mistake, and we'll feck up your education and do our best to increase your chances of ending up on the streets with your nose in a bag of glue".
Drugs are everywhere - you can't shield your kids from them forever. Many, if not most, young people have tried drugs, and a number of them may go on to become occasional recreational users before getting more staid as they get older.
Alas a not insignificant number of these people will get addicted, and that is pretty grim.
I would add that, in my book, there is no difference between alcohol and other recreational drugs. Indeed, alcohol causes more harm through its ubiquity. But banning the sale of alcohol doesn't solve the problem - and causes many new ones (viz prohibition in the US and attendant gangster-related crime).
I think it's important to keep some sense of perspective - the vast majority of people who smoke the odd spliff here and there (and even some who smoke every day) do not go on to become hardened addicts of class A drugs who will mug you or burgle your home. Most people get bored and grow out of it. A small minority will become addicts. I also believe that propensity to addiction is at least partly something that people are born with - in my experience anyway.
Moral panic never solved anything, nor did heavy-handed punishment.
PS I think I know what you're hinting at mcnab - sometimes these things happen - regardless of the "quality" or otherwise of the source.