I'd definitely print something off from the internet that gives them the facts about cannabis, and I'd make a point of sitting down with them and talking about your grades and how you're doing well and intend to keep doing well.

I don't know if I'd try and talk to them about your use of cannabis, not yet anyway. Also, seeing as to how they responded really well to finding out that you're smoking, I'd respect their wishes (to a certain extent) and not smoke in the house when they're home.

If the stuff you print off, and the grades chat goes well I'd try to talk about how you're using cannabis responsibly, but be careful - if you start getting annoyed and shouting, or whining, then you're undermining your "responsible" argument.

Good luck.