I find the lacked empathy from some posters towards an 11 year old girl really odd on this thread.
Of course most of us old mothers use tampons, and clearly there are some advantages of tampons, but the reality is that there are at least 2 standard sanitary options and the world over, many girls and women use towels - they aren't stupid or being pathetic, just making a valid choice. And understanding that particularly for a young girl, for whom periods are new and perhaps not keen on tampons seems pretty easy to grasp. No doubt she will move beyond feeling like that, but it's how she feels now, and that's fine - it's not immature or silly for a young girl to feel anxious about putting things up her vagina when she's never done it before, even if more experienced women can say it's not a big deal.
And yes, some mothers do actively discourage their young DDs from using tampons. Personally I would be offering all alternatives and leaving DD to make a choice, but I think that if some mothers feel 11 is too young and it's best to wait a while, then that's their choice to make. It's not possible to say categorically that tampons are always the best choice for everyone - so why do some people ridicule or belittle the choice of an 11 year old to use towels? Baffling.
Yes, simply write a short note that your DD can show the teacher. It won't be long before most of the girls are in a similar position and she feels less awkward about it. Feeling awkward about bodily changes is entirely normal for teenagers - again, it shouldn't be that hard to have some kind of understanding of that, even if most of us are long last that phase.