Your teacher was incorrect, and you should have challenged that mark. Yes, quotations are always supposed to be exact, although if the original contains a spelling error, the proper thing to do is include "[sic]" right after, to indicate that you know it's misspelled.
In your case, however, it wasn't a misspelling, and the teacher was wrong. She should not have docked you for that.
I had a junior high teacher who got visibly upset with me in class for challenging her for saying I'd made mistakes in an essay. I remember writing, "the trees lose their leaves" (referring to leaves falling from the trees in autumn) and she corrected it to "loose."
Well, that's obviously ridiculous. The tree has no conscious decision in this; the leaves are lost no matter what the tree might prefer. So I challenged her, and she muttered sarcastically, "I should have known you never make mistakes."
Funny what a person remembers, 40 years later. That wasn't the only argument we ever had, or the dumbest. She docked me marks for capitalizing "Earth." Since I'd chosen the solar system as the subject for my assignment, of course I capitalized Earth (back in those days I'd just discovered Star Trek and a renewed interest in astronomy, so I was working space-related themes into as many assignments as possible).
I asked her why she had docked marks for that, when she hadn't docked marks for capitalizing "Saturn."
"Well, Saturn is a planet," she told me, as though it was the most obvious reason ever.
"So is Earth," I told her.
This all played out in class in front of the other students, and if looks could kill I'd have become a puddle of goo right then and there. That teacher did not like to be challenged.