OP, I could have told this same story last year this time. My daughter was also 12 and going into 7th grade and was just devastated. I had prepped her, but it didn't matter. She was so upset.
What it came down to was she was embarassed, didn't want anyone to know, and was certain people would be "able to tell."
The first weekend, this is what helped.
Ibuprophen for the cramps.
Shopping.
My daughter loves to shop, but we don't go often just for her. She enjoyed it and it took the edge off her mood. We invested in some dark bottoms because she was fretting about leekage and stains. I also bought her a cute Vera mini-hipster for her supplies. I also bought a wide selection of products so she could chose what works for her. I know I'm picky and there are varieties I hate, so I wanted her to be able to choose for herself.
We also window shopped just for fun, checked out Claires, the makeup aisle and her other faves. It took her mind off the issue, but gave her a chance to deal with it in a public place instead of hiding in her room. (I think it helped her get over the idea that "everone will know".)
In the long run, what has helped the most was:
Tampons. Specifically the type where the applicator has to be extended before use. When they are in the packaging they are super small and you can fit them in your pocket (easy to carry to school and you don't have to carry a purse or run to your locker for supplies in the middle of the day). I bought some for our Disney vacation (which was exactly one month from her first period. ugh) as a "possibility" and as soon as she saw how small and compact they were she was determined to make it work.
Finding out that she is NOT alone. My daughter was convinced she was the only one of her friends to get her period. I was pretty confident she was wrong, but that's not something you just ask people.
By the end of 7th grade, they were all being more open about it and have their own code for talking about it. I think it helps to know you're all on the same team and there is someone you can ask if you need "help." Hopefully your daughter will open up to her friends, or they to her, and she'll feel better.
Good Luck. She will "get over it" eventually, she just needs some time and understanding.