It's not like I'm a grade-schooler with 〜じゃあるまいし

Grammar Explanation: It's not the case with 〜ではあるまいし

Seeing a frozen puddle on the ground, Nishikata is excited at the thought of picking it up and holding it. Takagi asks if this is what he’s thinking, and he quickly denies it.

西片(にしかた):: 「そんな…　小学生(しょうがくせい)じゃあるまいし。」; "It's not like I'm some kind of...grade-schooler."
高木(たかぎ):: 「ふーん。ホントかな。」; "Hm, I wonder..."