How does an immersion school of English work?

What can you make of the term immersion? Should the immersion school's maintenance crew along with the hall receptionist and even the cleaning maid, including its clerical personnel, all speak English in a non-situational way?

As escolas de imersão de línguas funcionariam bem assim? This question won't go away, no matter how hard I pose it.

Comments accepted both in English and Portuguese.