Personally I would take a middle of the road type of response. Not simply saying it is no big deal, but then again not treating it like a huge issue either.
I think that for the most part smoking pot is less dangerous to one's health than smoking cigarettes. EXCEPT that pot is not legal and that adds all kinds of ramifications such as the possibility of being prosecuted, the possibility of getting a drug mixed with other (worse) substances, the danger of being involved in any way with people who are illegally trading drugs, etc.
I would probably talk to my teen and make sure she understands all of the practical reasons why smoking pot could be trouble (well, I have had this talk already with my kids and it crops up from time ti time anyway--but I guess if I were in the situation described in the OP I would need to reiterate the message).
I would also want to know more about what is really going on. Are they growing it themselves just for their use? (thus taking away some of the worst concerns). Does she feel she is addicted and may need help to stop? Does she go to class or drive high, or is she only using after school when walking home? How does SHE view what she is doing (is she defensive and clearly trying to rebel? Is she trying to fit in with friends? Does she truly find it relaxing? Does she NEED to get high or just do so when the opportunity arises?).
My reaction would depend largely on what I found out from this talk. I MIGHT seek treatment for her (for addiction or an underlying depression, etc). I MIGHT take away driving privileges (certainly if I felt she may have driven while high). I MIGHT want her to only be places with adult supervision (but she could still be with her friends as long as a trusted adult were also there).
I would not try to tell a child (especially not an older teen) who they can and cannot be friends with. Nor would I take away a ton of stuff and set up a me vs. them situation.
I'm also probably the only mom of a teen that wishes the drinking age was 18 again. (which it was when I was young and dinosaurs roamed the earth

)
. . . .
I think condoning it as no big deal to 15 and 16 year olds..is the wrong message to send. I wouldn't condone alcohol for kids that young either. Are you going to supply them a list of drugs that are acceptable, and a list that aren't?

...Really, where do you draw the line.

Again, it's been my experience that so many kids turn to drugs, as it's readily available and so EASY to obtain. Certainly drug dealers aren't asking for their ID.
The legal age to drink beer and wine where I live is 16. I think that is great. The kids are learning how to handle drinking, how much is appropriate, etc. while they are still young enough to be living at home and have a parent's guidance.
You made some great points too - this is my thing, as I mentioned above. If you allow one drug, and not others, you've lost all credibility as a parent, not to mention, you look like a fool. Teens love playing their parents for fools, and this is a great example of that. Seriously, will you supply your kids a list of what substances are acceptable or not?
Tiger
While I don't actually supply my kids with any such list, YES, I do think my kids (and most) are perfectly capable of handling shades of grey and knowing drugs (like most other thins) are not an all or nothing thing. I guess the drug/substance list for my kids would include this type of thing:
alcohol: never okay when driving. Otherwise fine (maybe even healthy) in smaller quantities but can be dangerous if not used appropriately.
cigarettes: legal (after 18 in the US I believe) but deadly to you and those around you (slow, painful death) and highly addictive. I hope you avoid them always.
Glue: use it to glue things (be sure there is good ventilation) but not to get high this can really be quite dangerous.
Caffeine: Fine in smallish doses, but don't get so hyped up on it you are shaking and don;t use it excessively to short change yourself on sleep.
Pot: Not all that bad for you in truth, but like alcohol it could be easily overused if not careful AND because it is illegal it is fraught with other issues. If you are in a country where it is legal and you know the source is okay, trying it in reasonable quantities is probably fine.
Prescription drugs: mostly good when used according to doctor's orders. Pay attention though and stop and get help for weird reactions and also pay attention to the research for anything you will be on long term. Drug companies are out for profit and not always as honest as they should be.
Meth: Could be deadly the first time out. Stay away from this!!!!
and on and on and on (you get the idea, right?).
Oh, and no I do not think I look like a fool when I allow my child to drink a Coke when they are sleepy and need the boost to make it through a late night show (they are in theatre) but not to smoke crack. I think I would look like a fool if i failed to recognize that different drugs have vastly different uses and reactions.