Language is the vehicle of education and the transmission of culture. Teachers especially need to value their native language and its literature. It is not enough to be a math teacher or P.E. teacher. All teachers must be English teachers.
When I taught high school math, I would require correct spelling and good grammar from my students. When they complained, I told them they would need good spelling and grammar all their lives as well as good math skills.
I agree completely. If we don't teach students standard English, then we're doing them a great disservice, for they will not have the skills they need for higher education and to compete in the job market.
Insisting that children and young people learn to use standard English appropriately in school is not the same as saying there's something wrong with speaking otherwise at home and with friends. Part of their education must be to learn how to use appropriate language in different contexts.
Mona, In all my writing about standard English I make the point that no one dialect is "better" than another. All forms of English are correct to their speakers, but being able to use a standard dialect in addition to one's home dialect is a social and economic asset. As you say, we can choose which form of English we want to use according to context.