剖析：Schools must be in session a set number of days each year.

Schools must be in session a set number of days each year.

请问这个句子怎么样剖析它的成分？什么是表语？句子的意思是？

Schools 主语

must be 谓语动词

 in session  表语

a set number of days each year. =...days per year/ every year/ per year/ ?a year 状语

可以理解为名词作状语, 或者是for介词短语作状语, 持续期状语前面的for常常可以省。

意思: 学校每年需要维持确定数目的上课天数。