操场(cāo chǎng) - playing field, sports ground in Mandarin

操场 (cāochǎng) is the playing field or sports ground, the open space at a school used for PE, break and running. 操 (cāo) is "to drill, to exercise" (as in 早操, zǎocāo, "morning exercises") and 场 (chǎng) means "an open ground, a field", so together they mean "exercise ground".

It covers anything from a plain dirt yard to a full running track with a football pitch inside it, so context does the narrowing. 在操场上 (zài cāochǎng shàng), "on the playing field", is the phrase you will hear for almost anything that happens outdoors at school.

一个操场 (yí gè cāochǎng) is "one playing field". A school usually has just the one, shared by every class.