To me the terms ‘Sir’ and ‘Miss’ are unequal because ‘Sir’ is a term of respect whilst ‘Miss’ has less gravitas as it has connotations of youth, juniority and marital status and can be seen as diminishing to adult women.
This has bugged me for a while, as I think it is exactly the wrong message that young people should be receiving in school.
A perfect alternative may not exist, but surely we can do better than this?
YABU - Sir and Miss are fine, no problem here
YANBU - Sir and Miss are too unequal and we should seek an alternative