This is a great question as Harper Lee actually uses a false climax in this Chapter to lull us into a false sense of security before the real danger reveals itself. This chapter then deals with the events before, during and after the pageant. Note how from the start the darkness is emphasised through the absence of any moonlight and the "sharp shadows" that are cast on the Radley house by the streetlight. Also, reference to their previous childish beliefs in scary stories is referred to, even though it is said that their belief in such things was now past:
Haints, Hot Steams, incantations, secret signs, had vanished with our years as mist with sunrise.
However, when they start walking across the school yard in the pitch black, Cecil Jacobs leaps on them to try and scare them. This is the false climax that Lee uses to give a deceptive sense of calm. It is when Jem and Scout walk back home after the pageant however, that suspense is built. Jem thinks he hears something in the darkness, and this is something that makes both Scout and Jem afraid:
"Thought I heard something," he said. "Stop a minute."
"Hear anything?" he asked.
We had not gone five paces before he made me stop again.
"Jem, are you tryin' to scare me? You know I'm too old-"
"Be quiet," he said, and I knew he was not joking.
References to "the stillness before a thunderstorm" and the stopping and starting serves to create great suspense as we, like Jem and Scout, wonder who is out there and why. As they keep on moving and they are sure that "shuffle-foot" is following them and they can hear him draw close, suspense is raised to fever-pitch, until this figure attacks.
Therefore suspense is raised through the false climax used by Lee, and then by the darkness of the setting, the isolation of Jem and Scout and the sounds of the figure that is approaching them.