The parents and students I know in Woodstock are happy with the middle school and the Academy. The Academy is an independent school that serves as a high school for the area towns and also enrolls students from elsewhere (who pay tuition, room & board). Their
website would be the best source of factual information. They have a good reputation.
Pomfret and Thompson are home to some private schools (Pomfret School, Rectory School, Marianapolis Preparatory), but I have not heard much about their public systems. I imagine they are small, given the population sizes of these towns.
I would not group Killingly with the other towns you have listed. It has made the news for the wrong reasons a few too many times over the last 20 years or so.