it's reported that two schools ,both of which are being buil

把两个逗号中间的句子去掉以后,头尾两段形成一个完成的句子。这说明,中间那段不是句子,是一个从句,定语从句,修饰two schools。定语从句有定语从句的用法,two schools是先行词,which是引导词,因为有both of 这个介词短语在,所以不能用that。扯远了,你可以找本书详细研究下定语从句。