Currently, there is a function to disable sync to PowerSchool in the 3 dot menu next to the blue +Assignment button on the Assignments page.
This function disables all assignments from syncing to PowerSchool.

The opposite function is necessary for users that accidently used the disable function or never enabled assignments to sync during creation.
Consider adding this functionality or at least an API function that allows the option to enable sync to SIS during the creation and modification of assignments.