The ONLY cool ones was Buffy Spike and maybe Anya.
Xander Willow Tara Dawn and Reilly were not cool.
Good people, but not cool.
First let's get Dawn, Riley and Tara out of the way. WE don't know what they were like before. Tara could have been MUCH cooler than she was in highschool and I have to imagine that even as big of a pain as she was, being human kind of makes Dawn cooler than whatever she was before....or maybe not....hard do say about a ball of energy. Still, she's not their age so it doesn't matter.
Riley was at least cool for being "mysterious" army guy and we also don't know how big of a dweeb HE was in highschool...I'm guessing, pretty dweeby.
However, since the beginning, Xander went from someone Cordy wouldn't even bother listening to much less caring about to her "main squeeze" and since she's the coolest girl in school we have to imagine his "coolness" level rose. It waffled in S4, but he regained it once he got his construction job.
And Willow was dating a guitarist...which I hear boosts your coolness level. Plus, she also turned "friend" to Cordy after a while. By the time she hit college she had all those hip haircuts and clothes and "fit in" with the college crowd....hence her happiness on the first day of college.
These may not seem like GIANT waves of coolness, but when we see Jonathan in college, he's no more evolved than he was when we first saw him. And in S6...he's still a sniveling dweeb. (An adorable one that I love, but I've got a weak spot for sniveling dweebs as Willy well knows!)