Final answer:
Scout and Jem first suspect Cecil Jacobs is following them, but as the situation becomes more ominous, they fear it could be someone else with a more threatening intent.
Step-by-step explanation:
In the scene described from To Kill a Mockingbird, Scout and Jem suspect that they are being followed as they cross the schoolyard at night. Initially, they suspect that it might be Cecil Jacobs, a schoolmate who had previously tried to scare them. However, as they hear the sounds more closely, the suspicion shifts towards a potentially more ominous presence. The passage with the ominous signs and sound of heavy shoes is intended to create tension and foreshadow a significant event in the storyline.