the number of the students in their school is over nine hund

这里的主语是the number of ,表示“……的数量,”而不是“students”,因此谓语动词是用单数,is. 但是如果变成“a number of students"作主语的话,表示很多学生的话,谓语动词就要用复数.
比如：
the number of students in our school is 200.
a number of students in our school are fond of football.