Travel of less than 30 days to Canada, Mexico, or a Caribbean Island (other than Cuba): You do not need to apply for a new U.S. entry visa if you plan to travel to a contiguous territory for less than 30 days, provided that you have the required documents for re-entry to the United States. This is known as "automatic revalidation". Do not surrender your I-94 when you leave the U.S.; if you do so, you must obtain a new visa to return. Citizens of Iran, Iraq, North Korea, Libya, Sudan, Syria, and Cuba are not eligible for automatic revalidation. People with cancelled visas are not eligible for automatic revalidation and also people with single entry visas may not be eligible for automatic revalidation, as a single entry visa may be presumed cancelled. Finally, if you apply for a new U.S. entry visa in a contiguous territory, you may not reenter the U.S. using automatic revalidation.
从墨回美国有个auto revalidation的程序,具体和学校国际办公室确认。