I have 2 in American high school right now, 9th and 12th. Schools vary from place to place depending on local culture--so in Texas, (American!) football is absolutely huge, but in parts of the Northeast, people almost don't care and other sports like soccer and lacrosse are more popular. Some areas are ethnically and socioeconomically diverse; some are quite homogeneous. TV is not very accurate I would say.
It's definitely not bad to be academic at my kids' school here in Maryland and the kids work very hard. Schools are bigger: we have 2,200 kids in grades 9-12. There's all sorts of kids and most of them do find their niche socially. As well as sports there's all sorts of other activities at a large urban or suburban school: service activities, academic-adjacent clubs like Mathletes, Odyssey of the Mind, robotics club, Mock Trial or debate, and more social things like LGBT group, D&D club, Asian student club, etc. It seems that the number of clubs has risen in the past 20 years, and they've been the best way for mine to make friends. Most areas still run early, but there's a push to have high school start after 8 so kids can get a full night's sleep. California passed a law requiring that.
Some places have a dress code but not everywhere (ours is incredibly basic and bans bare midriffs and visible undergarments) and schools tend to be less particular about appearance, jewellery, etc. DD goes to school in Doc Martens and coloured hair (a mix of blue and purple at the moment). I would expect that would be an adjustment for someone used to strict British uniform. The only shoe rules are closed-toes in science labs and sneakers for PE. In two states we've yet to have decent school lunches, so don't be surprised if your children decide to pack sandwiches. Based on my friends' experiences, I don't think that much has changed since your day in that respect! One thing that has is that it all seems to be digital textbooks now and they give the students laptops.
In my experience Americans think British people are cool and will love their accents.
Little things: it's funny because in many respects I think American teenagers are more supervised than British ones, certainly when comparing middle class kids, but within the school walls, I think it's a bit less so.