My recent assignment was to write an essay using a template my teacher had given me. The essay was supposed to begin with a phrase like 'It is common knowledge that' or 'there is no denying that', but I wanted to be more original and wrote the phrase 'It is no secret that', which I had heard used many times, instead. Later, my teacher told me the phrase was kind of obscure and most likely informal and that I shouldn't use it. So I want to find out if it is actually applicable and acceptable in an essay.
P.S. I'm not a native speaker, so please don't judge me if I'm being stupid.