Citizens of the United States are required to have a valid U.S. Passport in order to enter a foreign country and to re-enter the U.S. Some countries require that your passport be valid at least six months beyond the dates of your trip. Please check with the embassy or consulate of the country in which you plan to study for their specific requirements.
In addition to a U.S. Passport, most countries require a visa for extended stay. In order to get a visa you must have a valid passport. If you already have a passport but it expires before the required dates of validity, you will need to apply for a new one. To check individual country requirements for visas you can connect to the Embassy website: www.embassy.org/embassies
If you already have a passport and if that passport was issued within the past 12 years, after your 18th birthday, you are eligible to apply by mail. If you do not meet these requirements, you must apply in person. If you have an old passport that has expired and it was issued before your 18th birthday, you must apply for a new passport in person as if it were your first.