"From the union perspective the development is worrying," Fjelkner tells the daily Svenska Dagbladet.
She links it to the charter school reform which allowed private companies to run schools but still be funded by taxes - meaning many more schools now vie for prospective students' attention.
"We've seen almost an explosion in the number of charter schools," Fjelkner says. "And it means the market is saturated."
Looking at the Swedish high schools, Svenska Dagbladet reports that their marketing budget has doubled in the past three years.
The amount of money spent on sending out flyers and other ways of luring students to pick their schools is equivalent to the annual pay of 83 high school students, the newspaper notes.