The English teacher was always mean to my brother and I/me.
The principal and she/her always picked on the boys in the class.
We/us boys hated both of them.
We can see which pronoun to use if we mentally take the noun out of the sentence:
The English teacher was always mean to I/me.
[S]she/her always picked on the boys in the class.
We/us hated both of them.
Without the noun, most people will quickly recognize which pronoun is correct.
The English teacher was always mean to me.
[S]he always picked on the boys in the class.
We hated both of them.