The "control" group didn't go on any kind of field trip. They just continued to attend class like normal. So there's no reason to believe that the art had any influence. It could just be that giving kids a day off from the usual school grind, getting them away from their usual neighbourhoods, and showing some kind of interest in them beyond the norm had a positive impact.
I do happen to believe that exposure to art can aid in personal development, but this study does little to prove that.