Undies and lies... More adventures in potty training

All summer we attempted to get Jack to use the potty with no luck. The boy just has no interest. SO I was pleasantly shocked and surprised to learn that Jack has been going on the potty at school every day! He's been averaging using the potty twice a day while at school. This led me to believe that Jack would use the potty at home this past weekend. However, I couldn't have been more wrong...
On Saturday morning Jack told me he wanted to wear his new big boy underwear. He assured me that he would "not pee on Thomas (the Train)" so we gave the underwear a try. He sat on the potty several times but nothing happened. I was doing dishes in the kitchen when I noticed Jack struggling to carry his Mickey Mouse upholstered chair into the living room. He set it down and the following conversation ensued...

Jack: "The chair's wet"
Me: "Why is it wet buddy?"
Jack: "I don't know"
Me: "Did you pee on the chair?"
Jack: "No" as he walks away back into his room

I, of course, followed Jack back into his room where he again denied that he had peed in his underwear and on the chair. I checked his underwear and not only had he peed in it he had also gone number two! When I told Jack what I found his response was "Oh, I did?"
I cannot help but wonder why he uses the potty at school but not at home. Could the peer pressure at school be responsible for Jack's occasional potty success? If so, how do we harness the power of positive peer pressure to use at home? I'm running out of Resolve and Febreeze.