No, I do not believe that students at risk that drop out of school do mainly because of drug use. I think that it has to do more with their home situation and whether they the motivation from their parent or family and if they have the drive within themselves. Drugs may be one reason for drop out, but I do not think that is the main reason in any way.
Many kids that drop out of school come from areas that they know they will never get out of. They think they are not smart enough to make it through the system. They know most of the system could careless about them. So their mindset is, why should I attend school when it is going to do nothing for my future? Many of them feel lucky just to make it to the age of 18. It is not always drug use that is the problem. Many of them sell drugs, but do not use them. Others do use drugs to make the reality of their situation bearable.
No, many students that are at-risk tend to have no support system at home. Many factors contribute to dropping out such as not being good at school, living outside the home causing for the need for a job, and some with or without the cause of drugs. Many times school makes students more at-risk than if they were to just have a job.
I do not believe all at-risk students that drop out of school do so mostly because of drug use. I believe this scenario is common, but it can't be used for all students because it's simply not true. While I was in high school I know a few students who quit because they already had a decent full time job and they didn't see any reason to continue. I think it's important to note that these people, 12 years later, are still stuck in low-wage employment and they are more than likely making less now then they were 10 years ago.
I think that most people who drop out of school are not necessarily doing so because of drug use, but probably mostly because of other factors like self esteem and home life. Drugs definitely can be a contributing factor, but I do not think it is the main one. Also I think drugs are more of a problem after the student drops out and then gets really addicted.