[e-Newsletter] Email newsletters by group

We are an elementary school and we are looking for software that would allow the Principal and PTO Board, for example, to be able to send newsletters to the entire school membership list but also we have classroom coordinators that we would like to just be able to have rights to email certain subsets of the entire school (grades or classrooms). We already have a WordPress site for managing school projects and volunteers so a WordPress plugin would fit us perfectly if it has these features. Do you know of a plugin or combination of plugins that could do this for us?