I spent three months in Italy, living with a family and attending (for part of the time) a local high school; I'd like to reiterate Benji's point about making sure you consider how much support you will be getting from the program.
I went through an au pair agency that placed girls with families in Ireland and Italy, and they had an Italian language school in Rome. Within a week, I'd gotten a call back from the director with a family profile--they were interested in having me for a year. I read over the contract, and was struck by a clause about visas. I emailed the director, who said not to worry about it, that none of her girls ever had visas. I worried anyway, but as far as I could tell on the Italian consulate website, I'd be fine for the first three months on a tourist visa; then it looked like I could obtain a student visa from the American consulate in Milan, so long as I'd signed up for some classes.
The first hint I had a problem was at the airport, where the lady at the check-in desk gave me a funny look when I said I didn't have a return ticket. I explained I'd be living with a family and going to school. Were they relatives? No. Did I have a visa? Well, no, I thought I could go on a tourist visa-- No, only if you have a return ticket within three months.
So I had to buy a return ticket there, on the spot, for her to let me board the plane. But I still thought there was no way the au pair agency would mislead me like that.
I did end up getting into classes at a local high school in Italy, so I emailed the American Consulate in Milan and the Italian Consulate in Los Angeles. Both agreed; the only way I could get a visa was by going back to L.A. It would've been nice to know that before leaving.
There was a list of required documents for a student visa on the LA Consulate website, so I got everything on that and made my appointment. Three months later, back in L.A., I finally got up to the window and explained what I had and what I needed. She explained in turn that there was a different list for minors. I was surprised--there had been no information to indicate such on the website, or when I'd talked to the American Consulate, or when I called the Italian Consulate to make my appointment... evidently, they are not allowed to make such information available online. The only way it can be obtained is from visiting the consulate. And the list for minors required me to be living either with a first-degree relative or at a boarding school. So living with friends and attending a high school was not okay, even though I had a letter from the principal of the high school and another one from the family.
And basically that was the end of my Italian adventure. It was a wonderful three months, but the program I went through was much more fly-by-night than I'd thought--the director called me up when I was back in the US, saying I should have just incurred a visa overstay, that it "is not a big deal." Maybe not, but I didn't want a fine and a black mark in my passport (and the threat, if I incurred again, of being banned from the Schengen states for 5 years).
Since you're going through the UC system, I would hope that there wouldn't be these sorts of problems. But always do your own research, and be careful who you take on their word.
