1. Buy a Korg TM-50 and a Peterson pickup from woodwind brasswind for EVERY kid in your high school band. Preferably every kid in the program. Use the Instructional Materials Allotment (IMA), formerly the textbook fund, to pay for these. There's even a technology subsection of this fund. They run about $23 a kid, total. We use them and have for several years and it has made a HUGE difference.
2. Even if you can't get those, have a pitch reference and set each student's horn length. I have the pitch going, have the kid play their tuning note, then I help guide them to playing in unison with the pitch (I point down if the kid is sharp, having them bring pitch down, I point up when the kid is flat to bring pitch up). I do this with EVERY kid during concert season, every day, and 1/2 the band every day (woodwinds one day, brass the next) during marching season, as needed. I don't do it every day because we have about 130 in the high school band and we don't have time every day, especially given that everyone has a tuner. Very quickly you will start to see kids adjusting and matching better. You have to guide them because no amount of explanation will suffice. It's experiential learning at its finest.
3. Have a harmony director and use it to play "problem chords/notes" as needed. One of the biggest positive changes has been the ability to have a problem pitch played and then have the kids play with it, matching. This also helps with chords that you want to start talking pure vs. equal. The Harmony Director can do both. Again, we ordered ours through the IMA allotment.