Buffy Quibble

OK, it's minor, but this one keeps bugging me: At the beginning of Season 1, Buffy was 15 when she started Sophomore year. At the beginning of Season 7, it is implied that Dawn is starting Freshman year, but she is 16. What gives? Sloppy writing? Was she actually taking high school classes before, but in a random building? In fact, for those three years, where were all the teenagers going to school? They could have spent two lines explaining all this, but instead these questions are left to haunt me. *sigh*