You want to modify the default security that is applied to objects instantiated from a particular structural class.
10.19.2.1 Using a graphical user interface
Whenever a new object is created in Active Directory, a default security descriptor (SD) is applied to it along with any inherited security from its parent container. The default security descriptor is stored in the defaultSecurityDescriptor attribute of the classSchema object. If you modify the default SD, every new object will get that SD, but it does not affect any existing objects.
10.19.4 See Also
MS KB 265399 (HOW TO: Change Default Permissions for Objects That Are Created in the Active Directory)