The teacher remained po-faced throughout the serious meeting.

po-faced 是一个形容词，由 po（可能来自 poker face 中的 poker 缩写）和 faced（脸）组成。po-faced 形容一个人表情严肃、不苟言笑的样子，通常带有贬义，暗示缺乏幽默感或过于刻板。