Many AT&T IPHONES for StraightTalk conversion

My Grandfather purchase about 40 Iphones 3GS,4G and 4S, with ATT. We are wanting to resale this to help with our Cheer expense to go to the Orange Bowl .

Many students want them to be used with StraightTalk.

What is the best way for us to allow StraightTalk to be used? Or is there a service which isn't too expensive which can do that for us.

Thanks Katie,