Children with single parents at my school were very much in the minority.

minority 是名词，由形容词 minor（较小的、次要的）加上名词后缀 -ity 构成，表示"少数"或"少数群体"。在这个句子中，in the minority 是一个固定搭配，意思是"占少数"或"属于少数派"。