After you are done with Ghirahim, go to the Sealed Grounds and be ready for yet another boss fight.
After you finish the fight, open up the gate of time, cross over and talk to Impa. Once all the cuscenes are done, head to Skyloft. In the academy, talk to Gaepora. When you're done with him, talk to Owlan in the office next door. Finish the spiral charge training and head to the Lumpy Pumpkin, an island southeast of Skyloft. Talk to the owner, then head for Thunderhead, the cloud ball where you learned all the songs. Land on the rainbow island to start another boss fight.