Not everyone is as strict as you are. Here are some reasons one might allow this:

1) high schools sports are focused on participation
2) the tights are not a safety issue
3) the tights are not flashy or showing off
4) "religious reasons" can be scary to some officials

Bonus story: I was the middle for a high school soccer game a couple months ago. I spotted a girl with tape over her ears about 5 minutes into the match. I asked her what was under the tape and she replied earrings. I properly went and yellow-carded the head coach. The head coach said "the earrings are permanent and for religious reasons. I decided to let the girl play. I was really torn about my decision for a couple months until I ran into a really really good ref. I asked him about the situation and he said he had also worked at that school this year and had the same situation. He also allowed the girl to play. Incorrect by rule, yes, but I'm not losing sleep about these situations any more.