Complex sentence: Each girl in turn stands and tells Mrs Kingsley that they were with Poppy at the time of the fire thus sabotaging the court proceedings enraging Harriet into disclosing the actual cause of the fire.
Split into simpler sentences: Each girl in turn stands up and tells Mrs Kingsley that they were with Poppy at the time of the fire thus sabotaging the court proceedings. The show of support for Poppy among the pupils enrages Harriet into disclosing the actual cause of the fire.