"We finish school at 3 tomorrow."
I think it’s the last three characters.
完学校 sounds really odd to me. School is this context tends to just be 学 or 学习 or 班 （下班）.
And finish is also more likely to be 下 (to get off school) or 放 than 完 (to finish or complete school).
Your sentence might mean that you’re constructing a school (the premises) and you have completed it.
But that’s just my impression from what I’ve heard over the years. I’m not a native speaker and there are regional things too.