My ex girlfriend used to wear heels all the time, in school, going on dates, hanging out, etc. In my head I thought "whats the point, wouldn't shoes be more conformable?". I don't think it makes girls look sexy, classy, or trashy. I don't think much of it.
I'm going to go with sexy and classy with a bit of preppy bitch ;P. I liked it when I was in high school, I think that it makes a girl look confident and like she puts a lot of thought into her clothing choices. If she likes looking good for herself then she'll like looking good for me ;)
I don't classify wearing heels as any of those, but personally I think there's a time and place to wear heels and I just don't think school is the place to wear the, so I think the girl is just trying too hard. Could be because I'm more of a flats/sneakers type of girl.