The basic premise is very simple: the teenaged offspring of Lovecraftian horrors go to high school with humans. Wackiness ensues.
The ensuing wackiness is very important: in a show of complete disrespect for ol' Howie, mindblasting, world-devouring horrors from beyond space and time are played completely for laughs. You will save the Homecoming game from the rising of Cthulhu, use Elder Signs to fend off the products of the school cafeteria, travel through abhuman realms of twisted geometry in search of forbidden knowledge, and do it all in time to make it to Speculative Civilizations class in order to ask out that cute Byakhee who sits in the back row. Gibbering insanity is far less worrisome than being grounded, and being attacked by a multitentacled semi-corporeal monster is just an inevitable consequence of being caught necking with its daughter.
The default setting for SCOOS is HP Lovecraft Memorial High School, in the New England coastal town of Arkham. The natural enemies of HPL High students are the yuppie scum from Whately Preperatory School in the expensive suburb of Dunwich, and the riffraff from Obadiah Marsh High in the disreputable town of Innsmouth, just down the coast. Plus, of course, the staff and faculty of HPLH itself. Parents and siblings are also common foes. Against these threats, you have only your native wits, a few supernatural powers beyond human ken, and, if you're really lucky, a library card.