Management of Schema is restricted to members of the Schema Administrators group.
Additionally, schema modification requires the ability to write to the schema. This is disabled by default.
To enable a schema administrator to write to the schema, use Regedt32 to navigate to:
HKEY LOCAL MACHINE\System\Current Control Set\Services\NTDS\Parameters
On the Edit menu, Add Value name Schema Update Allowed, as a REG_DWORD data type. Set the data value to 1. It is not necessary to restart your server.
NOTE: To disable the schema updates, set the Schema Update Allowed data value to 0.