English teachers are on my list, right next to librarians. Way at the top of the list. I adore them all. Here are some of the last bastions of civilization, and considering everything is an uphill battle for them (for the good ones) I am proud when they accomplish anything.
That said, like emeraldcite said, just because she's an English teacher doesn't mean she knows the first thing about writing a story, or how the mechanics of a story work.
An eye doctor is still a doctor, but I wouldn't trust him to get inside my brain. So please, don't say "just an English teacher," go talk to her and find out why she replaced the commas. Talk to her. Have opinions, for god's sake have reasons behind them. Talk, debate (politely, please) and find out that your English teacher is probably a wonderful person, very intelligent, doing the best she can.
Even if she can't punctuate your dialogue correctly (Neither, I should point out, can seventy percent of young writers) (neither, I should point out in honesty, can I some of the time) I bet there's still a lot of stuff she can teach you.
That said, remember when her opinions start going to different places than what you know about writing, consider them and consider them carefully, but do remember that optomotrist ain't neurosurgeon.