With up to 15mph winds, the slow-v or slowstick would be very bad choices. At those wind speeds, the plane would hover against the wind.

Some of the cheapest planes you will find here at raidentech:
http://www.raidentech.com

The Wind Dragon is only $100 RTF and takes around 15 min to put together using no glue:
http://www.hobby-lobby.com/wingdragon.htm

If the purpose is to provide each student with his own plane to own then you have to go cheap. Otherwise would likely be best to buy say 4 EasyStar ARF's ($240) total. Then buy 2 TX's, and enough RX's, ESC, servos for the 4 planes. Then either buy lots of batteries (15 or so) or buy 3-4 batteries and a peak charger that can charge in around 15 min. I would expect no more than 1-2 will be flying at the same time.

Having recently learned to fly myself. I would say it's best to learn basics of aerodynamics/flying & dangers of flying/plane safety, then do Sim training, then learn how to build/setup plane properly for level/stable flight (correct CG), start with basic turns, etc. Later increase throws for some loops, etc.