I ran into the same design barrier a while back for a running studio.  They wanted all the control from a portable device vs. walking over to hardware.  I spent a while looking into hardware with app control which was the wrong path.  I came across an app called Anytune Pro which did everything I needed.  They were able to build their own playlists in the app and adjust speed and pitch on the fly.  I used Airplay to a Airport Express from a dedicated iPod touch for each instructor, don't use a phone!

Make sure you have a solid network (Ruckus is my choice) and make sure you inform them that getting all the licenses needed to play their own music is their responsibility (ASCAP, BMI, etc.)...... put it in the contract, make them sign it!