There are different ways of structuring menus illustrated in various skins, like you see on this site we have 2 levels of horizontal menus and then anything deeper is shown in a vertical sub menu. A similar menu structure is illustrated in the andreasvicklund02 skin.
Probably what you are trying to do could be done with javascript in a vertical menu but it would be easier if you had a horizontal menu with High School, Middle School etc, then a vertical sub menu below would naturally only show children of the current page.
If a menu contains multiple pages at the same depth it naturally wants to show them all so probably a javascript solution would be required to hide the other pages at the same depth as the current page.
Hope it helps,
Joe