Currently isn't possible for Former Students to be visible within the Parent Portal, and this causes a lot of missed communication and over work with families. It will be ideal to leave former students in as a "read option" for their grades, into parent portal.
If you go into the Parent Portal Manager
Stay on the Current Status Tab
From the menu on the right, pick "Homepage Features"
One of the tick boxes is "Enable show former children on the homepage"
That might be what you're looking for.