This comes up a lot but generally speaking I would steer away from online programs. There are MSW programs of varying prices throughout the country you can attend. Universities usually create relationships with local institutions in order to make sure you are placing for practicum and internship, both required components for a clinical MSW. Established brick and mortar institutions tend to make those connections better. Many also offer professional part time options specifically for individuals who are looking for options that will fit with full time or part time work.
As for your interests in working with adolescents in mindfulness based therapies, you can pretty much do that professionally after you graduate so long as you're working with that population. I wouldn't worry too much about the mindfulness piece so much as just placing in internships and practicums where you are able to work with that specific population.