Any staff or students (users) enrolled in Google Apps for Education will maintain access to that account and all associated Mail, Drive, etc. from year to year. Please arrange to take your Google data with you if you want it before you graduate or leave.
We do perform account purging at the end of each academic year. During the last week of the long summer break we will purge (delete) any accounts that will no longer be active in the new academic year, such as year 6 students who have moved up to secondary school or staff members that leave the school. (Excluding any staff members on long term sickness or taking maternity/paternity leave)
From Google's Data Liberation site:
The Data Liberation Front is an engineering team at Google whose singular goal is to make it easier for users to move their data in and out of Google products. We do this because we believe that you should be able to export any data that you create in (or import into) a product. We help and consult other engineering teams within Google on how to "liberate" their products. This is our mission statement: