Поставьте глагол-сказуемое во все временные формы Continuous и Perfect в страдательном залоге: The system of secondary education in the United States is known to be a conglomeration of many different kinds of institutions.
Is known was known has been known had been known will be known will have been known В Continuous глагол to know нельзя поставить
Ах да, еще would have been known)