How long should we keep student files?

Quick answer: 5 years.

There is no "hard and fast" rule. 5 years is the recommendation for keeping intake, exit and attendance data. It is particularly important to keep this data as it would be required for an audit (most likely 3 years).  In addition the MN Dept of Ed policy on documents retention says (not explicitely) that any education record that has any "stake" for the learner attached to it, e.g. GED, diploma, perhaps mandatory MFIP referral attendance data, should be kept permanently.