Tuning the ensemble is essential.  Playing in tune for an individual is simply not optional.  With advanced tuning applications available on smartphones - especially the Tonal-Energy tuning app (available for Apple and Android and used by many of the best high school and college bands in the country), it is critical that we take advantage of the readily-available technology in the classroom. This tuning app allows students to see a very precise measurement of their pitch.
One way we can actively use this technology during rehearsal is with a microphone connected to our phone's tunning app.   This will allow the tuner to ONLY listen to you during a live rehearsal, giving you very specific feedback in realtime!  Without the microphone it is listening to the entire band - which does not help the individual student.
There are a couple of microphone options.  By far the most popular model is the Peterson "pitch-grabber" for mobile tuner apps,  Another is the OnBoardResearch "AppClip".   Both are $14.99 and will work with the mobile apps.  Look at the input mini-jack to make sure it will fit your phone before purchasing.  
No smartphone?  No Problem.  A simple portable tuner like the KORG CA-2 personal tuner can be purchased local for under $25.00 (Korg CM200 microphone w/larger 1/4-inch jack to fit this tuner is sold seperate) and works the same way as the mobile app in rehearsal.