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 knownwas knownhas been knownhad been knownwill be knownwill have been knownВ Continuous глагол to know нельзя поставить