Vidigami accounts do not expire. As long as users have activated accounts, they can continue accessing Vidigami to the years they have been invited to even once they have graduated from the school.
The only way users would not be able to access Vidigami anymore is if the school decides to remove them completely from each school year; which most of our schools do not usually do. (e.g. deleting students in the enrollment page will automatically delete the parents for each school year)
Just a quick reminder, if the school opens up a new school year (2017/2018), they should not provide the names of the users who have already graduated on the onboarding form since they are not part of the new school year.