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.