There is no reason why a teacher should be forced to reveal their sexual preferences to their pupils. A teacher should be allowed to talk about their personal life with their students if they choose to do so, but there should not be an obligation in any form or context.
No, all teachers should not openly declare their sexuality to their pupils as the teachers' sexuality is of no consequence. A straight teacher, gay teacher, bisexual teacher, or any other sexuality teacher can teach just as well as anyone else, and the ability to teach is not impacted by sexuality.
No, I think it is no one business what your sexuality is, it has nothing to do with teaching, you are hired to teach not talk about sex and what your preference is, that has nothing to do with your agenda it should not be discussed in no environment not at all.