I had similar issues with my youngest son. His grades started tanking in 6th grade. He was smoking weed, drinking, and in general hanging with the bad kids.
Many teachers and the admin staff at his school lectured me and felt like I wasn't doing enough to curb his behavior. In truth, he was banned from all electronics and grounded to the house after school with the instructions that he could win back his freedom, phone, TV, and gaming privileges once I got reports from his teachers that he was turning his grades around.
Long story short: He did not turn his grades around and in fact, the GPA that earned him all of that went from .87 to .69 by the end of the school year. (What a brilliant idea my school district has in allowing middle school students to progress to the next grade even though they earned most Fs throughout. A hard thing to work against when you have a stubborn kid.)
During the time where he was grounded during that period, I picked him up from school for an orthodontist appointment. I was about 10 minutes late in getting him. I dropped him off at the ortho and got a call that I needed to come pick him up because he was vomiting. When I loaded him into the back seat of the car and started driving him home, he began singing The Doors' 'Whiskey Bar'. So. In the 10 minutes between being released from school and me getting him, he got drunk as hell. (Insert huge sigh.)
Now I'm looking at the kid and he's not the reason I joined this forum (waving hi! I'll get to my story later!) and he's 20 years old. He is a nice human being, helpful around the house and generally nice to have around and so I continue to allow him to live here. However, he won't go to college and has been unemployed for almost a year now. He is an alcoholic and I'm just getting to the point where I'm helping him at least own that he has substance abuse problems.
When all this started with both my sons, I asked my family doctor, an older gentleman with 5 adult kids, what I should do. He regaled me with tales of his own kids and their issues as they were growing up, and they were equally to worse as bad to what I was going through at the time. He said, "All you can do is to let them know that you love them and you'll still be on the other side when they come through their problems. From experience, if you try to force them to make the right decisions, they'll just get sneaky on you, so try to keep the lines of communication open". It was pretty good advice.
Looking at the issues my own siblings and I have gone through, the issues my kids have, I have nothing but support to offer. My parents couldn't figure it out. I couldn't figure it out. My Stanford graduate doctor could not figure it out. So. I keep letting them know that I love them more than anything and I'll still be here no matter what with my fingers crossed that they figure it out like most of my brothers and I did.