iPhone 4S does not connect to Airport Express Wifi Network

My iPhone 4S will not connect to my Airport Express network, but everything else will. My MacBook, Xbox, Apple TV all connect to it with no issues whatsoever. I have tried resetting Network Settings several times using several methods (i.e. reset both airport express and iphone at same time) and nothing works. This is exactly what happens: I go to Wifi, and slide to On; two networks appear, my schools wifi and my airport express network (which is hidden and password protected); I tap onto my network, the little spinning loading icon appears, disappears, flashes once and then my iphone just switches back to 4G.

I really want it to work because I like to AirPlay my XM over my Airport and mirror my phone onto my TV through Apple TV. this is driving me insane. My airport express is the latest generation!

thanks!