The formal or polite form of address is the second person plural, as it is in other languages. It hasn’t survived in Modern Irish, though (Iolra an ómóis).
Why leibh and not leat?
leibh (and any prepositional pronoun (what leat and leibh are called) ending in ibh) is the polite form.
leat is for people your own age/younger who you are quite friendly with.
I learnd "tidsear" for teacher. Is that wrong or are both correct?
Is it just me, or does thidseir sound like (using English spelling) "heedshuth"?