I'm a teacher in the UK and have 30 kids in my class.We have bought 35 (older) Nexus 7 tablets.
I have one Google account which I want to use across all devices so that the young children don't have to remember user names and passwords.
The trouble that I'm having is that not all of them are able to be logged in at once. There appears to be a limit as to how many devices can be online with this account at once. We're doing this to be able to use Google Drive so that the kids can produce work and save it into Drive.
We are finding that as tablets are booted up, already online device are logged out from Google and I have to log them in the android backend.
I'd love to get this sorted. It's for the kids!
This is as close to an answer as I can find at the moment. Looking at the site for the (soon to be launched) Google Play for Education site, one of the benefits that they mention is:
Android tablets in the classroom
Google Play for Education brings the innovation of Android technology into classrooms. Educators can set up and deploy large numbers of devices in just minutes or hours rather than days.
Over-the-air delivery to student devices
After finding apps they want to use, educators can push them instantly to student devices over the air. They can send the apps to individuals or groups of any size, across classrooms, schools, or even districts.
(My emphasis, from http://developer.android.com/distribute/googleplay/edu/about.html)
Which sounds promising, but if you go to the Sign Up/School Interest Page it has this interesting bit of wording on the form:
Which suggests that there's a limit somewhere around the 30 device number. Which would match up with what you're seeing. As I can't find anything else official mentioning this limit, I can't find any official advice for getting round the limit either, sorry.
The Google Play for Education Store is creating content optimised for Nexus 7 tablets. I would be amazed if this is not available for the older tablets. I have seen no dates yet for when the store will be available, but Google have an event in November and suspect it may be then (as the service was advertised for autumn). However, they are only soliciting interest from American schools, so it may not be initially available in the UK. In the meantime I would suggest maybe looking at TabPilot which will allow you to achieve what you are after as well as a lot of other useful functionality.