I clicked on this thread because DD also mirror writes, and while I've not noticed her doing this on the back of the paper, often there is some reason I could see that triggered the mirror writing (for example, having a crowded space on the right side of the page, so she starts from the left to better control the remaining space).
It's my guess that her mirror writing stems from something very perceptive - she apparently SEES very well, and has noticed that when looking "through" the back of a paper, the letters are all pointed the other way. So she's keeping things consistent, and this is honestly something you might actually appreciate and be impressed by. It is very perceptive of her.
I'm a homeschooler, so I don't know how to deal with teachers. I would take this particular thing into my own hands and work on this myself.
I'd have to think about this some more, but first I'd think of games that require knowledge of left and right. Is she right handed or left handed? In Kindergarten, we were taught "I write with my right" which is helpful. DD is left-handed, so I made one up for her "My left is my best." In the car, sometimes we ask her whether we should turn left or right (for a route she knows well). She has to think about it and she's not right 100% of the time, but she generally is getting the idea for left vs right, and the idea that they are different.
Make a sheet like the one she gets at school (even ask for copies if you want, or just recreate a similar setup). The back of the page has the last three spelling words. BEFORE SHE BEGINS, have her draw an arrow at the top of the page that points to the right. I'm thinking it would be best to then flip the page and draw another arrow pointing to the right, right away, before she goes back to the first page (but alternatively, she could complete page 1, and then on page 2 make her first step to do this arrow). Point out to her that the arrow she draws on the back side looks like it's pointing a different way than the one she drew on the front side if she were to see through the paper. Pointing that out might help her understand this and disregard her urge to make them point in what she sees as the same direction.
And then she writes the words going in the direction of the arrow.
At home, if she mirrors the second page, first just give her feedback on the words themselves ignoring the mirroring. She must feel frustrated, and it really sucks to be told you are all wrong when you actually know the words well. Did she write the words? Did she spell them correctly? If yes, note that so she can feel good. If not, I'd actually ignore it for now. The point is just to acknowledge her effort so she doesn't feel like giving up. (I'm not an overpraiser, I promise, but something that is clearly frustrating to a kid needs extra grease). If she mirror writes, tell her "great, now just write them IN REVERSE" and this will be perfect.
If it helps for her to think about the letters being backwards on the back of the page, let her. What I'm trying to say is, a typical instinct would be to tell her she's writing it backwards, but she does not see it that way, she thinks she's writing the correct way. If it helps to just say "listen, on the back of the page you write the other way" (even if you don't see it like that) then that works. Later she will figure out that you write the same way on both sides, but meet her at her current perspective for now.
I don't know if I'd try anything with the teachers - it would depend on your daughter's feelings about the grades, I guess. If she didn't really care, I would just let it be and work on it at home and not have a thing to say about the 70%. If she cared a little, I'd just tell her not to worry about it - that you are pleased with her work and you can see she is doing better than the teachers are giving her credit for. If she cared a lot, and working with her at home didn't have quick results, I'd go to bat and try to get the teachers to give feedback on the spot or give her partial credit or the opportunity to verbally spell the words she got wrong or something.