Same here !
My son is 14, and smokes cannabis at least twice a week.
I bought some home drug testing kits (£5 each online), and test him regularly.
He says lots of teens smoke weed , and it is easily available.
I have tried the sit down chat, a referral to a local substance misuse charity, and directing him to the FRANK website.
Problem is he thinks it's harmless,
(Which it probably is in small amounts),and nothing anyone can say or do will convince him.
Am worried that his twice weekly habit will spiral, and school work will suffer.
( At moment she is doing fine at school)
One tactic that does work, is to severely ration their access to money.
No easy answers ,I'm afraid.
PS- worth remembering that for the majority of kids , cannabis doesn't ruin their lives, and most go on to lead happy,productive lives.
Just hope mine, and yours, fall into that category.