Is there any way of communication with students? I haven't found any tool for sending messages or even emails (though I'd rather not use this one, as emails have become passe recently ;-) and only about 30% of my students have any email account). I'd really love to send some comments to my Ss...
There isn't. That isn't what Duolingo's for - Duolingo provides resources for people to study, and tools to let you monitor some of our students' use of Duolingo.
You'd need to find another way of communicating with your students - your school's VLE is probably the best way of doing so, or via your school's email system.