Plugin to send messages to members/lists but not full-blown newsletters

I'm looking for a plugin that would allow the owner of a school to send student members short email messages. We don't need a full-blown newsletter feature but it does need to be able to have multiple lists (kids classes, adult classes, etc). Users don't need to talk to each other, just get a message that, for instance, the 10am class is cancelled, etc. Something like mailchimp seems like overkill and too much work for the client. Ideas? Thanks!