The Turnigy and FlySky systems on HobbyKing work just fine for FT park flyers (I went through both of these radio on my way up to the awesomeness of the Taranis). From a transmitter perspective, it's really hard to beat the cost and functionality of these - $25 and $45 for radio/receiver combo, and additional receivers are between $6 and $12 each.
http://www.hobbyking.com/hobbyking/...Ch_Mini_Transmitter_and_Receiver_Mode_2_.html
http://www.hobbyking.com/hobbyking/...HDS_Transmitter_and_6CH_Receiver_Mode_2_.html
The downside to these is HobbyKing international shipping and no compatibility with any of the BNF models you might run across or be able to get donated. The i6 model does have student/trainer support, but check the cable compatibility before planning to connect it to another radio model.
The other good option I see for you is the very cheap end of the Specktrum compatible DSM2 format. I have had great luck with the LemonRx receivers, and they are super cheap (around $6 each).
http://www.lemon-rx.com/shop/index.php?route=product/product&path=59&product_id=93 I have 10 of these now and haven't had a single issue.
These receivers will bind up to any DSM2 compatible transmitter, so there are lots of affordable radio options. For a radio with expo and dual rates you could go with the OrangeRx 6 channel much cheaper than any of the Spektrum brand transmitters
http://www.hobbyking.com/hobbyking/...10_Model_Memory_and_3_Pos_Switch_Mode_2_.html
But the little Parkzone / Hobbyzone / eFlite MLP4DSM transmitters that come with everybody's Ready to Fly kits will work with Lemon receivers too - and lots of modelers have these just hanging around collecting dust. They are all over eBay for $20 or less - asking around for donations could get you a box of them quickly too. These don't have any expo or rates controls, but for starting out you can limit the rates with the servo linkage just fine.
http://www.eflightwiki.com/eflightwiki/index.php?title=Spektrum#MLP4DSM
http://www.ebay.com/itm/E-Flite-Bla...959028?hash=item1eb020cef4:g:3mUAAOSwSclXK8BS
http://www.ebay.com/itm/ParkZone-PK...412790?hash=item2eeed4b436:g:sXkAAOSwRH5XJ7E2
http://www.ebay.com/itm/E-flite-HP6...054161?hash=item43edd64fd1:g:9ooAAOSwY3RXJkm-
For an instructor radio, you could get any one of the cheaper Spektrum models with a trainer capability, and either bind the student radio right in if the big one supports wireless, or build one of these cheap modules for around $15 to turn a wired trainer port into a wireless one.
http://forum.flitetest.com/showthre...ios-without-native-wireless-trainer-functions
For a programmable instructor radio the cheapest option would be the OrangeRx I linked to above, but the wireless buddy box would need to be wired up slightly different to match it's cable connection.
The big advantage to staying with DSM2 signals with a youth group is the compatibility with 90% of the Bind and Fly market here in the US. Allows planes, receivers, and transmitters to more easily 'fall into your lap' and be used by students with the rest of your radio gear.