Canadian Border Patrol officers (not the Mounties) only monitor people entering the country, not those leaving. It is incorrect that the kids would be inspected by the Mounties as they are about to leave Canada. Fez would've had a problem with US immigration upon reentering the US, not the Canadian authorities.
Fez is a foreign exchange student, so he has to be in the US on a Student Visa, not a Green Card.
The two Mounties interrogating the boys are wearing the formal red serge. These uniforms are not worn during regular duty, only for formal events.
When the guys are stopped at the border, a sign in the background lists the distances to several Canadian cities. Winnipeg is misspelled as "Winnepeg".
The original plan to go to Canada without Kelso doesn't make any sense. As established in the "Halloween" episode earlier in the second season, Kelso is the only one of the kids who's over 18 and therefore is the only one old enough to buy beer in Canada. If they had gone without him, they wouldn't have been able to buy any beer. Also, why make a 700 mile trip to Manitoba (assuming they're from the suburban Milwaukee area) when the drinking age in Wisconsin in the '70s was 18.
The map of Canada in the RCMP office shows Nunavut, which only became a territory in 1999. Nunavut was not a Canadian territory in the 1970s.
The guys sing "O Canada", and include the line "From far and wide, O Canada". That change was added in 1980, when the song officially became the National Anthem via the National Anthem Act.
Being outside the Kenosha, Wisconsin area the closest Canadian entry would be Windsor, Ontario, which is 350 miles away vs. Manitoba, which is 700 miles away.
The map behind the characters, when they are being interrogated by the RCMP, is of the Niagara Falls/Ft. Erie border crossing, between New York and Ontario.