Is it possible to install lower version os in ipad?
It is possible to downgrade to iOS5. You will just have to "trick" iTunes into thinking you are restoring from one of you backups.

Just make sure to first make a backup of your current settings. Then it's just a matter of restoring from the "fake" backup(IPSW-file).

You can download an IPSW(backup file) from http://www.maclife.com/article/howtos/how_downgrade_back_ios_5