Last year the number of the students in our school----------

选B.
你没弄清楚a number of ,the number of
a number of 许多,一些,大量的,相当于a lot of ,some,后面接名词复数.
如：A number of apples are red 许多苹果是红色的
the number of ……的数量 ,后面接名词单数
如：The number of the apples is 40 苹果的数量是40