Time to explain a pun I can't be assed to come up with an alternative for~

The name of the cultural festival is the Tenjou Festival (天上祭), rather literally "heaven festival". The joke Otaku is making here is that the characters 天 (ten, heaven) and 上 (jou) can be read the same way as the テン (ten) in テンション (literally "tension", but more means high spirits or excitement in Japanese) and the verb form 上げる (ageru, to raise). They then can be read as the internet buzz word テンアゲ (tenage, get hype), which is a shortened version of テンション上げる (tenshon ageru, get excited/raise spirits).

And that's your Japanese wordplay lesson for today.

A new girl joins in?

Guess Otaku-kun will be visiting class 3-1.