I used to be a specialist in the field of dysgraphia and dyslexia, back when it was pioneering work. As a teacher, I had a student in eighth grade who was failing and I decided to analyze what was wrong and found that she did ALL mirror writing. She was left handed as well. I asked her if anyone had ever noticed it and she said i was the first teacher who had ever noticed it!
The brain can do very strange things, and this is one of them. I'm pretty amazed if the school has not noticed this, since young children should be observed as they are learning to write so that they will form letters correctly. I would get your granddaughter tested by a specialist and it may be that she needs a special education plan.
My mother, who was a master reading teacher and taught first grade for many years, used to tell little stories to the children as they learned to write so they would do it correctly. I can't remember them all, but she might say "A little man wanted to go down town. He started off from his house and he went straight DOWN the path. But then he remembered he had forgotten his hat and he was so excited he jumped up so high he went all the way back in the air and landed at his door so he could grab his hat." (This was lower case "i.") I would think that once your granddaughter got a firm foundation writing the letters correctly -- and she would always move from left to right as she formed them -- it might help her start to form words from left to right as well.
Also, I would start her very early with correct "keyboarding" -- what we used to call typing. One of my adopted children did not do mirror writing but had a lot of trouble spelling words backward or just plain wrong (to would become ot if she was tired, and girl would be gril, for instance). I found out that sound was very important to her, so I told her she was like a tape recorder and she had to listen to the tape playing "in her head." I also had her use a typewriter (no computers back then). She would first look at the word and spell it either aloud, or "in her head out loud." Then she would say the word, and then spell it again. As she spelled it, she typed it. Taking the pressure off forming the letters correctly made a huge improvement in her spelling and writing. My daughter ended up getting a degree from Long Beach State and was on the Dean's list many times. She made her own little private 'spelling aids" that she used when writing, after getting permission from her professors.
You might want to check out Lindamood Bell, which is an excellent program for children with similar problems.
I have read some of the other replies now, and they all put it down to a temporary developmental problem. Yes, it could just be that, if it is only occasional. Perhaps I am over-reacting, but unfortunately, as I taught eighth grade, I found so many youngsters who had exhibited symptoms of reading and writing problems way back, and no one had noticed or done anything. By that time, they were entrenched as "losers" in the system and thought they were "dumb." It was so sad, and could have been prevented. So I would be very proactive with some supervised "game and story based" writing, keyboarding, and then get professional help if the problem does not clear up.