Complex sentence: She dances with Freddy at the school dance much to Harriet's dismay but Poppy trips and hits her head which leads Freddie to take the concussed girl outside for air where he asks her out on a date.
Split into simpler sentences: She dances with Freddy at the school dance, much to Harriet's dismay, but Poppy trips and hits her head. This prompts Freddie to take the concussed girl outside for air, where he asks her out on a date.