の used as a conjunction?


Can someone explain how の is being used in the following sentence? It seems to
be used as a conjunction to me, but I've never seen it used this way.

> そう、今では公立でも５年生あたりから英会話を教えるのが当たり前なんだよね。ちなみにうちのクラスの児童は全部で３２人。
> **内訳は男子１６人の女子１６人** 。

_（from 小西マサテルによる「名探偵のままでいて」）_

I assume the bold sentence means "the class was composed of 16 boys and 16
girls", but I expected the writer to use に instead of の.

