YANBU. Many schools in America do not allow any logo/branded items, bags or shirts or shoes, with the understanding that they have no place in school.
At some point you simply have to decide to parent, ignore cries of 'peer pressure," and choose your battles according to your values.
I remember as a kid when logo sweatshirts had just started coming into shops. Some friends of mine had ESPRIT sweatshirts, and I asked where they got them because I was sure they had to be a free gift with purchase. I couldn't then, and still have difficulty, wrapping my head around the idea that people PAY to wear brand logos. It wasn't always like this, and doesn't have to be like this. It's essentially consumer choice.
Good Luck!