A new study shows you can be easily re-identified from almost any database, even when your personal details have been stripped out.

Source: You’re very easy to track down, even when your data has been anonymized – MIT Technology Review

I see this in schools, where the staff will strip out any identifiable information from data such as grades, but there is still enough information to figure out who the student is. Since the teacher is usually listed, that already brings the number of possible students down to whatever the class size is.