Did Leo take Stargirl to the Ocotillo Ball?

No, Leo does not take Stargirl to the Ocotillo Ball. At this stage in the story, Leo has broken off his relationship to Stargirl, as he can no longer handle being a social outcast due to his involvement with her. Stargirl has become deeply unpopular at school, and the last thing Leo wants is for some of that unpopularity to rub off on him. Turning up at the Ocotillo Ball with Stargirl on his arm would mean instant social death; no one would want to have anything more to do with him.
Nevertheless, Leo does pop his head around the door later in the evening. But he does so under cover of darkness so that no one will see him. When he gets to the Mica Country Club, he sees Stargirl make a characteristically grand entrance. She arrives in quite some style, in her usual wacky attire, riding in a sidecar of a bike covered in sunflowers.