I can't get iPad to update to latest iOS

I am a teacher and I have been issued three Ipads to use with students. All of them are Ipad 2s. Two are updated to the latest iOS, but the third is currently at 4.3.5. I have connected it to my computer, hoping I could use Itunes to update it (this worked with one of the other Ipads), but it's not working. I understand Macs pretty well, but I'm a novice when it comes to Ipads. Is there something I'm missing? I've got a kid who's really frustrated because he can't download certain apps he wants until the Ipad is updated. I'm getting pretty frustrated too.