By broadcasting The Evil House before The Howling Ghost and The Haunted Cave (so Evil House, a Halloween episode with trick-or-treaters, would coincide with Halloween), a continuity error was formed. The Haunted Cave takes place before the school year starts, with Sally helping Adam submit his paperwork to Principal Blackwater - two months or more ahead of Halloween.
The human skeletons in the cave are medical skeletons. You can tell by the horizontal precision cuts around the skulls and the metal connectors keeping the bones together.