I think there were probably a few times when my Mom made me change my clothes before leaving for school, most likely saving me some embarrassment from the dress code enforcers. However, she was pretty hands-off, despite how weird my 80s style was sometimes, and I try to mirror that with my own kids. For me, it's one of the few areas of their lives kids can exercise some personal control and individuality.
The issue of appropriate school attire has been a hot topic in our area at various times, such as after an incident in the spring when a Redmond junior high student
The school shared graphic depictions in a pdf it created after students asked for visual representation of what was allowed and not allowed, but in essence, the school says underwear should be under clothing, no exposed buttocks or midriffs, and no alcohol or drug logos or profanity on clothing. Ultimately, the note says, school staff doesn't want to embarrass kids by asking them to change, but it will enforce the rules, and provide kids with alternate clothing if they don't have any spares with them.
The Issaquah District's official policy, slightly less detailed, can be found here. Lake Washington School District's official dress code is similarly open to interpretation.
Where do you draw the line--are you hands-off or directive in your kids' clothing choices?