Can't say from experience, as I graduated high school in 2004 (which made Mean Girls kind of iconic for people my age, as it was poking fun at what we were leaving behind, though TBH what the movie portrays is far more like my experience with middle school than high school) though I imagine that for them Mean Girls is kind of like me watching Heathers, which deals with similar themes but a different era of cultural references, and came out around the time I was born.
Movies about popular high school girls are kind of a genre of their own, and can be entertaining, though usually not very realistic. I think it comes from our societal fear of teenagers (see also: the teen witch genre), and specifically teenaged girls. One's teen years are a sort of liminal space in which one experiments with what can be socially accepted, and teens with power (ie popularity) can really take this to the next level.
What I find more interesting is the way these movies shape the way kids actually behave, and vice-versa. For instance, my friends and I would basically roleplay bratty teens after we saw Clueless. We weren't yet teenagers ourselves at the time, but it gave us certain bizarre expectations of what that would entail (satire was lost on our 5th grade asses). The glamor! The power! The things that never came to pass, and mercifully so!
