A github project, PowerAPI, offers a PHP library for fetching information from PowerSchool SISes, but this does not appear to have originated from PowerSchool. Is API documentation available from some official source?
Updated information about the PowerSchool API (Release verison 1.4) is available on the PowerSource. (https://powersource.pearsonschoolsystems.com/dir/7536?from=search). You need a powersource account to login to the support site.
The newest link to PowerSchool API documentation is https://support.powerschool.com/article/66558.
Please note, as Chris mentioned in the previous comment, that you will need a PowerSource account to login in and access this information.
UPDATE: PowerSchool API Documentation can now be found at support.powerschool.com/developer/#/page/home