Let's say I want to build a site for a teacher. They will want a feature that can keep track of student grades. A similar application can also keep track of customer records, sales, etc. Where can I get one of these?

Basically, what I want is a prebuilt database system that allows an admin to modify and maintain all data such as a teacher, and allows each user to view their personal information, such as grades.

Is there a free system out there that I can install to help me do this?