A nonprofit organization on NPSP needs to be able to track the high school a...

Use NPSP Affiliations objects to connect the Contact to the Account for their high school and college, use the Status field there to indicate if they are currently enrolled or are former students there.

Turn on Field History Tracking for the Account lookup field on the Contact record, use the Account lookup to indicate where they are currently enrolled and change it as they move on.

Set up a Campaign for each high school and college and use Campaign Members to connect the Contact to the Campaign. Change the values in Campaign Member Status from Sent/Responded to Current/Former.

Install the Salesforce.org Higher Education Data Architecture (HEDA) managed package and use Affiliations.