Restrict root level pages

[anderson2877]

I'm finding that even though a group only has access to a specific page that if they leave the Parent selection as "none" they are able to create a root level page.

i.e I have this tree structure

Home
--> Teacher1Page
----> page1
----> page2
--> Teacher2Page
----> page1
----> page2

If I give access for a group to Teacher1Page, they are allowed to edit/add pages within the Teacher1Page tree.  This is fine.  

My problem is if they don't specify Teacher1Page as the parent page, the page is added as a root page.  As below:

Home
--> Teacher1Page
----> page1
----> page2
--> Teacher2Page
----> page1
----> page2
TeacherNewPage

How can I restrict the user from adding these root level pages.