It's high time that the girl-----sent to school .

虚拟语气,从句be动词一律用were,下面是百度百科“虚拟语气”的部分解释,具体你可以自己查一下
虚拟语气还可用在定语从句中,表示：“早该做某事了”时,定语从句中的谓语动词须用虚拟语气,其虚拟语气的结构为：It is (high / about) time that + 主语+ 动词的过去式/ should + do（优先使用动词过去式）,即从句用虚拟过去式.
It is time that I went to pick up my daughter at school.
我该去学校接我的女儿了.
It is high time you should go to work.
你早该上班了.