You want to change the default ACL for an object class in the schema.
Open the Active Directory Schema snap-in.
In the left pane, browse to the class you want to modify.
Right-click on it and select Properties.
Select the Default Security tab.
Use the ACL Editor to change the ACL.
Each instantiated object in Active Directory has an associated structural class that defines a default security descriptor (defaultSecurityDescriptor attribute). When an object is created, the default security descriptor is applied to it. This, along with inheritable permissions from the parent container, determines how an object's security descriptor is initially defined.
Recipe 14.12 for comparing the ACL of an object to the default defined in the schema, Recipe 14.13 for resetting the ACL of an object to that defined in the schema, and MS KB 265399 (HOW TO: Change Default Permissions for Objects That Are Created in the Active Directory)