Abroad 101, Higher Education Entrepreneur, International Education Marketer
If you study for less than 90 days you won't need a visa if you have a US Passport. For foreign passports, rules vary, check with the Italian embassy web site in your home country for details.
If you are going for longer, than 90 days you will need a visa and you will need to have this before you go. They won't let you into the country without one, odds are your airline won't let you on the plane without it. If you are going for more than 90 days and are in a group, the visas can be processed for the whole group without an appearance at the Embassy/Consulate. If not, you'll have to schedule and interview. Again, check with the Embassy website for more details or this trusted industry reference: http://www.nafsa.org/Content.aspx?id=20315