Some parents and drug prevention officers say not enough is being done to keep teens away from a major marijuana festival that lures thousands of students across Canada.

April 20, widely known in marijuana culture as "4/20," is a day when thousands of people gather for "smoke-ins" in cities across North America.

Some parents of Vancouver high school students are particularly concerned because the city’s school board has scheduled a professional development day for many teachers, meaning several classes are cancelled and students are potentially free to attend the annual pot-smoking event in the downtown core.

The coincidence angers Inge Mueller-Langer, the parent of two teenagers.

"I don’t think it's very responsible. I think the school should instead be educating our children about the potential long-term effects of smoking pot when you are a teenager," Mueller-Langer told CBC News. "This isn’t the same stuff we had in the '60s and '70s."

CBC News has obtained absence reports from the Vancouver School Board that show an additional 900 students skipped class on Wednesday, April 20, last year, when compared with Wednesday, April 13.

Mueller-Langer said she will not allow her two teens to take part in the downtown Vancouver gathering.

"If we had an event where children were being offered tequila shots, we would all be in an uproar. So, why are we not upset about pot smoking?"

Police stand by and watch

In past years, Vancouver police have attended the event, but uniformed officers have stayed on the perimeter and not made mass arrests.

"At the 4/20 event, conducting enforcement on minor possession of marijuana is not our priority," said police spokesman Lindsey Houghton.

Drug prevention experts like Tibor Palatinus, of Narconon, say students have no business at a public gathering where police stand by as thousands get high.

"It’s not a festival," Palatinus said. "It’s a pot promotion. It's a weed-selling promotion. Sponsored and promoted by drug dealers. That's what is going on."

The head of the school board’s drug prevention program, Art Steinmann, said parents need to talk to their teens about the 4/20 event, but admits it might not be an easy conversation.

"The parents that take too hard a line, that are too strict, that often isn't very helpful," Steinmann said. "And the parents who are so laissez-faire and casual and hands-off, that's not helpful either.

"We need to find that middle ground where parents are firm and have discussions and dialogue. Don't expect to always agree with your youth, and do things to help keep your youth safe."

Steinman said that although there are no classes, the Vancouver School Board is co-hosting a free concert all afternoon at East Vancouver’s Rio Theatre in an attempt to discourage students from heading to the 4/20 event.

The name 4/20 tradition has many reputed origins, but it's believed to have originated in the 1970s at a California high school where students would gather at 4:20 p.m. every weekday to smoke pot.

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(A larger image of this graph can be seen here)

With files from the CBC's Natalie Clancy