Reibun
、。。

After all, we're practically under house arrest in the school building from morning to evening. Of course you'd perk up once you're released.

Literal

After-all morning [from-から] evening [until-まで], school-building [inside-の中で] house-arrest state [reason-だからな]. Released-[conditional-れば] energetic [even-にも] become [casual-ってもんだ].

A humorous comparison of school to prison. 軟禁状態 ('house arrest state') — 軟禁 'soft confinement' + 状態 'condition' — is deliberately dramatic. 釈放 ('release from confinement') continues the prison metaphor. ってもんだ (contracted というものだ) means 'that's just how it is' — presenting the conclusion as self-evident. The whole passage is a student wryly rationalizing post-school energy.