Windows Server 2012 is not tested or supported on S3420GP, as this board is several years old already, and I don't think there is future plan to test or support it.
You can always try to install it, it's quite possible that Windows Server 2012 should have most of the drivers embedded already. However if you encounter any error, support from Intel will be very limited.
See Windows Server 2012 Compatibility and Tested operating systems