Konohamaru could [i]barely[/i] cast the Rasengan though. 
Sasuke was taught Chidori by a Hokage-level ninja, Kakashi. Not only that, Sasuke has the Sharingan. He can automatically learn ninjutsu as an Uchiha. 

As for how long they've been out of the academy I am not sure. There's also the fact that your character is a little older than the rest of the characters.