Part 1 - The US Government Requirements:
Students are encouraged to check with the Student/Exchange Visitor Visa Center before leaving the United States to ensure they have the correct paperwork.
To exit and reenter the United States on a student visa, the visa holder must be a returning student (i.e. possesses a valid I-20 form), with a valid student visa with multiple entry (indicated by a capital 'M' under "entries" on the visa, and a valid passport (containing at least six months of validity) with a valid I-94 (Arrival/Departure) card affixed inside.
Ultimately, U.S. Customs and Border Protection (CBP), a branch of the Department of Homeland Security, has full authority/regulation over the exit and re/entry of persons to the United States. To contact CBP with additional questions, please call their national customer service line at (877) 227-5511, or visit them online at www.cbp.gov/.
Public Inquiries Division
U.S. Department of State
Bureau of Consular Affairs
The Canadian government requires assurance that the student has the ability and intent to leave Canada at the end of the event so that they do not get stranded in Canada.
If their intention is to return to the U.S. at the end of their event, the government will be verifying the following at the point of arrival.
Also, if they are under the age of 18, a Canadian Border Services Officer may request a letter of authorization from their legal parent/guardian.
Website for Canada - Check this website to find out if you require a Temporary Resident Visa for Canada - www.cic.gc.ca/english/visit/visas/asp