Those who plan to visit Canada, Mexico, or adjacent islands, may re-enter the U.S. within thirty days on expired visas as long as they possess a valid I-20/DS-2019 and valid original form I-94 card unless they are excluded from automatic revalidation, as noted below.
Anyone who has applied for and been refused visa issuance at a border post is prohibited from re-admission to the United States in the same visa category, even though they are in possession of a valid I-94 form. Aliens who are citizens of countries on the State Department’s list of State Sponsors of Terrorism are prohibited from re-entering the U.S. using solely an I-94 form if their visa has expired.
Citizens from State Sponsors of Terrorism countries must obtain a new visa (and thus go through a new set of interviews, computer checks, etc.) rather than re-enter the U.S. using solely their I-94 form.
This benefit also applies to nonimmigrants who have changed their nonimmigrant status in the U.S., whose visa is still in the category in which they had entered the U.S. In this case, the visa is considered “converted” to the proper visa category as well as “extended” to allow re-entry.
22 C.F.R. 41.112(d); 8 C.F.R. 214.1(b)(1)-(4)
for detailed information.