According to Plesk 12 Release Notes MySQL 5.6 is supported http://download1.parallels.com/Ples....0-for-linux-installation-upgrade-info.html#6
But Plesk doesn't ship MySQL, it only downloads it from original OS vendor repo and it is quite likely it may be outdated there (especially for older OS). If your OS comes with MySQL 5.6, then Plesk would run MySQL 5.6 for clients.