Gibbon v10.0.00 has a bug, such that when editing school years it returns an error about unique values that is false. This has been fixed for v11, and the fix is backwards compatible, so please download the file below and upload it to /modules/School Admin:
https://raw.githubusercontent.com/GibbonEdu/core/v11.0.00/modules/School%20Admin/yearGroup_manage_editProcess.php
Apologies for any inconvenience caused!