She's a real estate agent who calls herself Mississauga's "#1 schools advisor," a trusted hand for parents who want the best education for their children. But the Peel District School Board says she has offended teachers, principals and students in a quest to attract business.

Meet Susi Kostyniuk, who promotes herself as part saleswoman, part statistician – a cross between jeweller Russell "The Cashman" Oliver and pollster Nate Silver. Her website, Susihomes.com, is replete with what she calls data-derived assessments, sometimes harshly delivered, of the performance of every public and Catholic school in the suburb of Mississauga, and many other suburban communities west of Toronto. (Take, for example, her view of Brian W. Fleming Public School: "Quite obviously, there are better odds for your children probably anywhere in Mississauga.)

Although it's not unusual for real estate agents to post test scores on their websites, Ms. Kostyniuk, has gone two steps further, devising her own methodology for ranking schools and then offering her candid opinions, often on video. Her system, she says, is supposed to take into account socio-economic factors to make the rankings fairer, but instead she has sparked a firestorm on websites popular with educators. While she is applauded by the likes of the Fraser Institute for trying to measure school performance, lawyers with the Peel District School Board are discussing how they can persuade her to cease and desist publishing her ranking system. "I think we're going to appeal to her sense of good taste and respect and ask her to not do this to our schools," said the board's director of communications, Brian Woodland.

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Ms. Kostyniuk declined to comment for this article, citing a barrage of complaints that she received after her website was recently featured on a Facebook forum for Ontario teachers, We Are the Frontlines in Education. "I've been up all night responding to them. It deterred my focus, focusing on my clients," she said Sunday.

Her rankings rely primarily on the standardized tests administered by Ontario's Education Quality and Accountability Office, but with a few twists. In an effort to identify underrated schools, she created what she calls the Teacher Difficulty Index.

While filming herself in promotional videos outside many of Mississauga's schools, she says she encountered teachers and principals who revealed to her the four main factors that make a teacher's job more difficult: lower household income levels, parental education, the number of single parent households in the neighbourhood and the number of ESL students. She purchased data about these factors from a polling company, and using a formula – she previously worked as a geomorphologist, her website says – came up with a list of schools that she believes are environments where it is more difficult to teach. From there she developed a "potency list" – schools that perform better than they should given the socio-economic factors in their neighbourhood.

Though he could not delve deeply into the accuracy of her methods, Peter Cowley of the Fraser Institute, said people like Ms. Kostyniuk should be applauded for trying to measure the performance of schools. The institute, a Conservative think tank, publishes an annual ranking of schools in Alberta, British Columbia, Quebec and Ontario based on their standardized test scores.

"There's a substantial need from parents for information about how schools do as it relates to where they want to live," said Mr. Cowley.

But many teachers and Mississauga residents who live in neighbourhoods that received poor rankings disagree – especially over what Ms. Kostyniuk says constitutes a difficult teaching environment. One online commenter, Mark Newby, an executive at a footwear company, said in an interview that, as a divorced father, he found her criteria "incredibly misguided."

"That comment about single parents, the suggestion that the children would be different from any other kids, was incredibly insulting," Mr. Newby, 43, said.