are teeming with It means the place is full of ..

全是有两 个意思，一，人很多；二，人人都是做相同的事情（锻拣）

有两种可能： 1. 操场上很多人在锻炼，没有多余的地方了。（有点夸张，是为了强调人多。） 2. 操场上的人所做的事情相同——都是锻炼，没有人在做别的事情。（例如晒太阳、盖房子、扫地、随地大小便。）

“All” can be used to express it.

i think it is a item which the author want to express his feelings, he exaggerate the truth, in fact, there may be some people, or many people.