Importance of High School/Prep School Alumni Associations?

Like anything, it depends.

My high school alumni association (I don't think there is one actually, but for the purposes of this post I'll pretend there is one) would probably only be useful for getting jobs digging ditches, picking garbage up off the street, and that is probably about all.

I use my school networks all the time, not so much for jobs, as for making business connections.

I've never personally used it because I don't live in the city where I went to high school. however, if I did, it might be good since I went to a prestigious high school and they always have professional networking events for alumni. It might be a lot of work to keep up, but in a smaller city/community, it probably wouldn't hurt.