用a,因为schoolbag书包的第一个音标是si是辅音书包英语怎么读，例如:l bought a schoolbag for my daughter yesterday.我昨天给我女儿买了一个书包。

this is a red schoolbag.这是一个红色的书包。

the boy likes a blue school bag.男孩喜欢蓝色的书包。

good luck.