I teach middle school and I have tuners on all my stands for my students. When students are playing warm ups the directors walk around and remind students to look at their tuners. Just because a kid has a tuner does not mean they are in tune. I have a kid that watches her tuner all the time and I still have to tell her to adjust because it is not in tune with the band. I use a drone when we warm up as well. In beginner band I play basic chords with my kids which helps train their ears to hear what a balanced chord sounds like. It is interesting when I put up tuners and the kids don't have them they ask about where are the tuners. Being able to see that they are flat or sharp does help fix it when you address it. However students will not automatically fix their tuning you have to teach them how to fix it. The tuner is just one tool that we use to help fix it faster.