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.)

Good luck !