Sub pages appear in the order that they are created, but this isn't necessarily the order that they are best displayed. It is easy for a teacher or administrator to re-order the sub pages as required.
Looking at the parent page, click on the Page Tools icon and select 'Manage Sub Pages' from the menu.
Drag and drop the pages into the correct order. You will see that subpages can become subpages of others as required (more on this).
Close the window when completed.