Trello is an excellent classroom app. I use it for student collaboration, posting assignments and rubrics, sharing class photos, adding links, creating a resource list, preparing for outdoor ed trips, checklists for a day or event, socratic questioning sessions.
In my second year of using it, there has been a marked evolution in our writing.
The @comments allow me to ask direct questions of a individuals or groups - perfect for quick chats or debriefs.
balzac-bee about Trello: organize anything!, v3.4.4