Google Teacher Center - join
Google and teachers have a similar objective: to help people build knowledge by providing greater access to useful information.
Low activity, 35302 members

Schoolsk-12 News - join
This group is for any information related to US public and private schools.
Low activity, 4 members

k12.chat.teacher - join
Casual conversation for teachers of grades K-12.
Medium activity, 816 subscribers, Usenet