Should I use different models for authentication

I have a rails project which is going to have two types of users - Staff and Parents.

In general, the website will have two different sections where parents can view student details and another where staff can update student details.

A staff may however also be a parent.

What would the best strategy in this case be -

Seperate model for Staff and Parents
Single user model with an is_staff and is_parents fields to separate the two