If B.I.A. SchoolSys showing ONLY_FULL_GROUP_BY error message on your server that means on your hosting ONLY_FULL_GROUP_BY is enabled in your mysql SQL_MODE. This issue comes due to on some hosting server mysql is not configured properly. Please contact to your hosting provider to disable/remove ONLY_FULL_GROUP_BY in your hosting mysql SQL_MODE or follow the below instruction to fix this issue (it will require full root access of your mysql server) –
Open phpMyadmin of your server then go to Variables tab then edit sql mode variable value and remove ONLY_FULL_GROUP_BY.
Note: This fix is temporary and if your mysql server will be restart or update it will comes on default state. So ask your hosting admin to make it permanently in mysql config file.
Generally this issue occurred in cheap or non professional hosting. If you will go with hosting providers like godaddy or hostgator or any reputed hosting provider then you will not face this issue.