What languages should be taught?
Spanish has a lot of advantages - it's a major global so would be very useful. It's also a very common language in the US which means both that it's immediately useful in daily life and employment and students don't have to search hard for opportunities to practise outside the classroom - at least in some parts of the country Spanish-speakers are easy to come across and Spanish-language media is widely available. It's also one of the easiest languages to learn for a native English speaker.
The most important thing, though, is not to wait until high school to start teaching languages. Teach kids foreign languages as soon as they start school - 4 or 5 of whatever it is in the US. Start then and you'll end up with some people beginning bilingual as a result - start in high school and the effect is negligible.