When is a high school going to have a domed stadium for football?! Wait, Am I off topic? Woops, never mind.
I think you mean which was the first domed high school football stadium. Two choices;
http://www.maxpreps.com/news/article.aspx?articleid=ad5c15d8-f6f3-4f13-8b27-d79fc7f4c9f8&page=7
http://www.smoaky.com/forum/index.php?showtopic=137748
Frisco ISD built a domed stadium. They then gave control of most of the dates to Jerry Jones and the Dallas Cowboys. Between the school district and the city, it only cost the taxpayers $90 million. 12,000 seats, so it is smaller than neighboring Allen's 18,000 seat open air stadium which only cost $60 million.