Revert "replace $db  = new WPSchool_DB(); to main wp-school.php file. This "global" variable will be use everywhere"

This reverts commit ea7d756fc29e71a9912813fc98592be603f58368.
Revert "Revert "replace $db  = new WPSchool_DB(); to main wp-school.php file. This "global" variable will be use everywhere""

This reverts commit 336d39f815b567bb8a47f24e9129a86e6abfecc5.
revert
revert