Hmm, I wouldn't be too sure about the pajamas as pants thing unless it is super common amongst HS students in your community. Even if it is, that doesn't mean that the girl is going to do it. I think you have to give her the benefit of the doubt.
Teens rarely wear the "top" of a pair of pajamas. Most pair them with a cheap tank top, or an over-sized t or sweatshirt depending on whether they are for bed or for lounging. Plus, if she is tall, it is common for the top to be uncomfortably short or boxy.
When I was in school, the only time I saw teens in flannel PJ bottoms was after sports practice (7 in the evening) or when athletes were traveling. We live in a rural area, and sometimes our athletes traveled between 3-10 hours to get to games. Although I still didn't participate and don't really approve, I find this type of wear far more acceptable, especially when likely the only people the kids saw were their parents and other teens. Then again, I live in a COLD rural area, so a little subtle flannel after dark is almost as acceptable as warm-up gear in public.
You said the girl was tall? Does she play sports?
You may want to try to pick a solid color or a subtle flannel in the school's colors.
