Complex sentence: Upon stopping back at Woodstock's tree Snoopy takes more notice of a set of footprints he had seen before; they lead away from Woodstock's tree, so the two follow them straight to the elementary school.
Split into simpler sentences: Upon stopping back at Woodstock's tree Snoopy takes more notice of a set of footprints he had seen before. They lead away from Woodstock's tree, so the two follow them straight to the elementary school.