As a teacher, it would be neat to see a messaging feature within classrooms. It would be nice to be able to send a chat or message to students either as a whole or individually for those students who need reminders or those who are excelling and going above and beyond their assignments!
Do you mean one way (teacher to students) or group chat?
If one way, perhaps you could use your school's email system if they have one.
If group, then perhaps zoom, Google Hangouts (both support audio/video/text chats) or Slack (text only) would be a good solution for you.
I don't know about where either of you live, but in my school district, students have google accounts but their email and google+ are disabled, so although that would be a good idea, it may not work in some districts.
My school uses Hapara with the Google platform, so I do have several other options, but I think it would be nice to keep it through Duolingo.