A junior student will often refer to senior students as "senpai", and alumni will often refer to alumni from earlier classes as "senpai".
On May 11, Taguchi and Ueda appeared and performed the song on their kōhai Hey! Say! JUMP's variety show, Hi! Hey! Say, while Tanaka was a guest performer on HSJ's other show, Show wa Hey! Say!, the day after.
("Waka-sensei" means "young teacher," and applied to Moriteru Ueshiba when Kisshomaru Ueshiba was still alive; this usage also exists outside of aikido, and is a facet of senpai and kōhai.)