Google Classroom is an excellent tool from Google that allows students and teachers to collaborate and communicate and allows teachers to assign work to students, collect that work, grade it and comment on it and return it to the student.
One limiting factor in using it was the fact that the students and teachers had to be in the same Google apps domain. Many schools use separate domains.
That has now changed. You can whitelist each domain in each admin console and allow them to work together in classroom. Simply follow the instructions here: https://support.google.com/edu/classroom/answer/6173514
It's easy to do and make Classroom accessible to even more schools.
Google Classroom Resources for Educators