If you try to extend the Schema for SCCM 2012 you may receive the following error messages:

Modifying Active Directory Schema – with SMS extensions.
DS Root:CN=Schema,CN=Configuration,DC=contoso,DC=com
Failed to create attribute cn=MS-SMS-Site-Code. Error code = 8224.
Failed to create attribute cn=mS-SMS-Assignment-Site-Code. Error code = 8224.
Failed to create attribute cn=MS-SMS-Site-Boundaries. Error code = 8224.
Failed to create attribute cn=MS-SMS-Roaming-Boundaries. Error code = 8224.
Failed to create attribute cn=MS-SMS-Default-MP. Error code = 8224.
Failed to create attribute cn=mS-SMS-Device-Management-Point. Error code = 8224.
Failed to create attribute cn=MS-SMS-MP-Name. Error code = 8224.
Failed to create attribute cn=MS-SMS-MP-Address. Error code = 8224.
Failed to create attribute cn=mS-SMS-Health-State. Error code = 8224.
Failed to create attribute cn=mS-SMS-Source-Forest. Error code = 8224.
Failed to create attribute cn=MS-SMS-Ranged-IP-Low. Error code = 8224.
Failed to create attribute cn=MS-SMS-Ranged-IP-High. Error code = 8224.
Failed to create attribute cn=mS-SMS-Version. Error code = 8224.
Failed to create attribute cn=mS-SMS-Capabilities. Error code = 8224.
Failed to create class cn=MS-SMS-Management-Point. Error code = 8202.
Failed to create class cn=MS-SMS-Server-Locator-Point. Error code = 8202.
Failed to create class cn=MS-SMS-Site. Error code = 8202.
Failed to create class cn=MS-SMS-Roaming-Boundary-Range. Error code = 8202.
Failed to extend the Active Directory schema.

The Error Code 8224 indicates either a connectivity or a replication issue between the domain controllers. Check and be sure that the secondary domain controllers are up and properly running.

After successfully extending the schema, ExtAdSch.log shows as follows:

Modifying Active Directory Schema – with SMS extensions.
? DS Root:CN=Schema,CN=Configuration,DC=contoso,DC=com
? Defined attribute cn=MS-SMS-Site-Code.
? Defined attribute cn=mS-SMS-Assignment-Site-Code.
? Defined attribute cn=MS-SMS-Site-Boundaries.
? Defined attribute cn=MS-SMS-Roaming-Boundaries.
? Defined attribute cn=MS-SMS-Default-MP.
? Defined attribute cn=mS-SMS-Device-Management-Point.
? Defined attribute cn=MS-SMS-MP-Name.
? Defined attribute cn=MS-SMS-MP-Address.
? Defined attribute cn=mS-SMS-Health-State.
? Defined attribute cn=mS-SMS-Source-Forest.
? Defined attribute cn=MS-SMS-Ranged-IP-Low.
? Defined attribute cn=MS-SMS-Ranged-IP-High.
? Defined attribute cn=mS-SMS-Version.
? Defined attribute cn=mS-SMS-Capabilities.
? Defined class cn=MS-SMS-Management-Point.
? Defined class cn=MS-SMS-Server-Locator-Point.
? Defined class cn=MS-SMS-Site.
? Defined class cn=MS-SMS-Roaming-Boundary-Range.
? Successfully extended the Active Directory schema.
? Please refer to the ConfigMgr documentation for instructions on the manual
? configuration of access rights in active directory which may still
? need to be performed. (Although the AD schema has now be extended,
? AD must be configured to allow each ConfigMgr Site security rights to
? publish in each of their domains.)