Sorry to read OP, I've had many a morning like this & it's horrible.
First of all this is obviously a notoriously tricky age and I would recommend doing some reading around the teenage brain / it certainly helped me.
Secondly - what else is going on? Why does he feel like he had to do or buy drugs? Bullying? Peer pressure? Stress about meeting other people's expectations? Struggling with the work so trying to avoid it altogether?
Thirdly - whilst sending him to a private school won't magically sort out his problems just because you didn't send DS17 private doesn't mean you can't send younger DS. Until you know what is really going on I wouldn't think this is the answer though. Private school is much like state school but with more frills and whistles - they still have to do homework.
Your older DS has witnessed this behaviour and should be mature enough to realise that whatever choices you make for DS2 are going to be in the best interests of the whole family. Speak to the current school - let them help you - education is a partnership between home and school.
In the meantime I would say that treating teens like this harshly never helps - I read something recently that really helped me with parenting which was working on playfulness, acceptance, curiosity and empathy - PACE - you can Google it! It's something used in fostering and adoption but it's useful for when the parent child relationship is strained or disconnected in all families and beyond toddlerhood imo.
Lastly, try not to take this personally, the sooner you can step away from the behaviour emotionally, the sooner you will be able to move forward. This isn't about you - and don't worry what other people will think of you as a family. Easier said than done, of course.