S1·E39 Fish School Musical

Jocktopus reveals that it is his father's dream for Jocktopus to sing in a musical play about potatoes. So during the Freshwater High musical play, "Potatoes In Winter," He goes in the play to sing but gets nervous.
Note: This is in two parts, usually listed as episodes 138 & 139. Disney still lists this as the season final, even after showing 3 more episodes after it.