The ever evolving and ever improving Google Classroom has just released an update that will bring a smile to teachers and schools who share staff or run cross campus teaching groups.
This update allows a teacher to work with a class of students outside of their base school domain. So for example if you were a specialist language teacher operating across three different schools you can now manage all of your Google Classroom activity from a single account. The other option I can see here is that teachers could also enrol for professional development run through Google Classroom from an outside provider.
For this to update to apply your Google Apps for Education administrator will need to add any external teachers to their white list to ensure security for students. Further information can be found here.