Archive your Photos Library

Over the course of a school year, the Photos library can get pretty huge, big enough that you really can't justify leaving it intact for another school year, without risking running out of hard drive space. A really quick way to have a fresh start is to just navigate in the Finder to the Pictures folder (Finder > Go > Home) and just delete (or move to a back up drive) the Photos library you see there.

When you reopen the Photos app, you will need to create a new library, but to be able to use web services like Photostream you'll have to tell your Mac to 'Use as System Photo Library'.