Not worth going to.
Those people with whom you didn't stay in touch ...
... there was probably a reason why you didn't.
Having said which, we have a good old girls network, and I suppose most schools are similar, and I quite often meet girls from my old school whom I didn't know when I was there.
It opens doors for you, but hey ... that's what schools are for.
Now I'm starting to change my mind about reunions. Maybe it is worth getting in touch with old school mates with whom you've lost contact.