ClassDojo added a messaging service to their free mobile apps. This week ClassDojo launched stand-alone messaging apps for iOS and Android. The ClassDojo Messenger apps are purely for messaging and don't include the other elements of ClassDojo.
Applications for Education
It appears that the purpose of the Messenger app is to make communication between teachers, parents, and students as streamlined as possible. Otherwise, I'm not sure why you would need Messenger when the original ClassDojo apps still have the messaging option included in them. ClassDojo's messaging service, like that of Remind and similar messaging services, hides the personal contact information of the teacher and the of the parents. Parents have to opt-in to receive messages from the teacher.