This is such an important gap to address. The linkage to care problem (as opposed to screening) is what you seem to be honing in on. You list several possible contributing factors -- do you think that the priority issue is not enough substance use treatment options for adolescents, or that these treatment options exist but they're just not being accessed?
If it's all of the above, then an intervention that attacks these various issues in combination may work best. It would be a big undertaking - probably best to be done in a limited setting/context first - but seems like focused/concentrated effort could be a form of demonstration project that could really demonstrate what factors are the hardest to work on or seem to be the most important.