there is a playground at our school为什么用at 不用in

there is a playground at our school为什么用at 不用in is,a,playground,there,big,our,school,in pay,acros There is Sports Day -------our school every term. A. at B.in There is a playground in our school怎么变成否定句 There is a tree____ the playground A at B near Is there a playground in youe school中文意思是什么 there is concert --- his school?中间用at还是in,为什么? Our school is at the foot of the mountain.there is a river i is there a play at your school what are the rules at your school为什么用介词at不用in There are at -------( little ) 2000 students in our school There are at ____ five hundred students in our school