I believe for the first two seasons they were using a lower quality film to save on costs? It used to really bother me in the graveyard scenes when you could barely make out what was going on lol, the show definitely got a lot brighter-looking in seasons 3 and 4. I think seasons 1 and 2 are a big reason why Buffy will never get a blu-ray release, they just don't have the HD footage from that time as everything was so grainy-looking back then

And hmm I still brought Buffy and Willow as high-schoolers,but Xander always looked to me like someone in his 20's who was held back :P And for Cordelia it was mostly the way they styled her that made it impossible for me to buy that she was still a teenager, they did seem to dress her a lot older that season and put her hair up a lot more. And they made an inside joke of it when Wesley assumed that she was a teacher at first