Complex sentence: She scarcely listens when Mrs. Upjohn, a parent, recognizes someone that she sees from her wartime days in the intelligence service, an oversight Bulstrode will later regret as there is a killer at the school who does not wait long to strike.
Split into simpler sentences: She scarcely listens when Mrs Upjohn, a parent, recognizes someone that she sees from her wartime days in the intelligence service. But there is a killer at the school who does not wait long to strike.