In Spinelli's Stargirl, Leo decides to allow Stargirl onto the Hot Seat show because by November/December of that school year, she had grown in popularity. "The change began around Thanksgiving. By December first, Stargirl Caraway had become the most popular person in school"(36). Hence, Leo had always worried that Stargirl would not have excelled on the show because she was too weird to comprehend--for himself and other students. After seeing her be promoted to the cheerleading squad, and everyone loving her at school, Leo finally felt more confident with her being accepted on the show rather than being rejected. Leo finally discusses his decision in chapter ten when he says, "All my resistance to putting Stargirl on Hot Seat vanished"(43).
Another reason Leo decided to put her on the show was because he believed in the show. From past shows, Leo had experienced only good feelings afterwards, even though the person on the hot seat was considered a victim. Leo's philosophy was that the show, "almost always added up to entertainment. At the end of the half hour, . . . there was always a good feeling in the air, and everyone--victim, jury members, studio crew--mingled and became students again"(56). Sadly, that wasn't to be the case for Stargirl. Leo explains that "in the month since we had scheduled the show, Stargirl's popularity had dropped out of sight"(57).
So, when he scheduled her on the show, everything was good and he could have predicted a good outcome. But things turned on Stargirl so quickly that Leo wasn't prepared for it, or he wasn't able to predict any type of outcome by that point. Leo did the best with what he knew and understood at the time. He probably could have pulled her from the show at any time before or during the episode; but true to his cowardly character, he didn't.