I studied abroad for a semester in Italy with AFS, and it was the best experience of my life. I was only 15 when I went, so it was really life-changing. Especially in high school, studying abroad gives you such an awesome advantage over your classmates in the USA. You'll return a completely new person, and having learned and experienced so much more than all of your friends at home. I lived with a fantastic host family in Gravina in Puglia, a teeny town in southern Italy, and I'm still in contact with them three years later. Italy is a really great country for study abroad, especially in high school, because Italians have such a warm and welcoming culture! They'll be just excited that you're there as you are.
Starting with a pre-departure orientation in the US, orientations in Italy, and usually the placement of multiple other exchange students in your Italian high school, you'll make the best friends you could wish for! I went to a regular Italian high school during the week, and made so many great friends. Because of the great public transportation system, you'll have a lot of opportunities to further explore both Italy as a whole and the region of Italy where you're living.
I had a full language immersion with AFS, and left Italy conversationally proficient in Italian. If you are able, study abroad in high school!!! It's such a great experience, and it really will change the way you see the world, especially at such a young age. I left Italy knowing so much more about myself than before I left, and am now studying linguistics and becoming certified to teach english abroad. Bottom line: STUDY ABROAD! If you can do it, DO IT!