Assignment completion - Google Classroom's main concept is around "course work" - i.e. assignments - which students turn in. Via the API, Classroom offers information on what assignments have been created and turned in.
It is theoretically possible to track assignment progress, as Classroom course work can move through a workflow and Google Classroom offers this data. Assignments can be returned or be in an "in progress" state.
Assignment grades are available.
Announcements - Classroom's other main instructional component is announcements; these provide a way for teachers to send messages to students. This can be tracked via the API, but seems less useful than tracking assignments.
Data Not available
Login data is does not seem to expose login information. However, we have heard that this is available via the general
Tools to Help
InnovateEdu has published a free, open source Google Classroom Connector. The tool works with a variety of data, but the most useful is likely it's tabulation of student completion of assignments.