I bet that there’s other UXO there there. Looking at Google Maps, St. Nicholas High School is on the beach, and about 1000 feet east of the Mataniko River. The mouth of that river was the site of a considerable amount of fighting between Japanese and American forces at Guadalcanal. After the Americans managed to defend the initial Lunga Point beachhead, which didn’t reach that far west, was maybe a mile to the east of that, that geographical feature was the next spot to establish a line at.
Video documentary talking about some of the land warfare at that point, with time offset for the start of hostilities relating to the area (using the spelling “Matanikau” rather than “Mataniko”):