Google's encrypted (SSL) search creates an obscured channel between a user's computer and Google "can bypass some schools’ content filters. This can make it hard for schools to stop students from accessing adult content," reports Google.
"One option is for schools to use SafeSearch lock feature, designed to help keep adult content out of Google search results. But given how many computers some institutions have this's proving impractical in many cases. So to prevent students from bypassing their filters, some schools are blocking encrypted search. However, a side effect of this action is that it also blocks other services hosted at Google's secure URL, including Google Apps for Education, and many of our other services which require authentication to keep information safe.
We're working hard to address this issue as quickly as possible and in a few weeks we'll move encrypted search to a new hostname – so schools can limit access to SSL search without disrupting other Google services, like Google Apps for Education," explains Google.