Any mothers of teenagers here?

I have a 17 yr boy & 15 yr old girl, as well as a few others, but these two are breaking my heart. I had so much pride and respect for my son with all his work and activities that he does. Then this weekend I cleaned up his room after a group stayed the night at prom. I find out he's got juules (I'm still clueless on these things) & heard stories of how he's had alcohol when at his events with his Ag/FFA group & heard stories of him speeding up to 100 mph on the back roads. Then today he had the gall to pull out a juule as he asked me if I wanted a juule then "puffed/smoked" it right in front of my face, in the kitchen while holding my 7 month old daughter. In all one swoop I lost all pride and respect for him. I've financially done so much for him to provide opportunities but to find out he's got no ethics, respect or morals is making me feel like I've royally screwed up as a parent. Where did I go wrong. I have a 21 yr old daughter too and we talked. She is horrified too. She said she never did that sort of thing and that I role modeled honest good character at all moments. My 15 year old isn't in much better shape. It's confusing though because I also have 11 & 13 year old boys and they see all this and swear they'd never do this stuff and the more they see their 2 older siblings do this, the more they step up to help me more as well as interact more positively with the other siblings and me. Any advice... Or just words of encouragement... I'm so disappointed and heartbroken.