The province was the last to eliminate Grade 13, after the 2002-03 school year, but the government still incorporates students who take a so-called victory lap in its official grad rate, despite criticism from the auditor general about the practice.
The government announced Thursday that the graduation rate for last year was 82 per cent, but admitted when questioned that only 73 per cent of Ontario students actually completed high school in four years.
"We don’t want to leave any of our students behind and we want to acknowledge that when you graduate, that’s an important step in your life," Education Minister Laurel Broten said.
"It means 93,000 young Ontarians have graduated from high school that haven't before, and that's something we should be so proud of."
Auditor general Jim McCarter took the Liberals to task in his annual report for using the five-year graduation rate as "the official measure of student success" instead of a four-year figure.
"Reporting the four-year cohort rate would provide an assessment of how many students have completed the curriculum — and how often schools have delivered the curriculum to students — within the four-year time frame," McCarter wrote.
The government had been using the five-year rate since the Liberals were elected in 2003, so have continued to use it despite reducing the curriculum to four years, said Broten. She declined to say if they would follow the auditor's recommendation.
"I'm cognizant that the auditor general has given us some advice in that regard and I certainly will consider his advice," Broten said.
The Progressive Conservatives argued the Liberals were simply not being straight with the public.
"It looks like the government cheated on their test score here, and are using a five-year plan on a four-year curriculum," said Opposition education critic Lisa MacLeod.
"They're cheating. They got caught cheating by the auditor general, and now parents across Ontario are going to see they were fudging the numbers."
New Democrats also criticized the Liberals' math, and questioned whether Ontario's decision to drop Grade 13 was the right one.
"The government is happy to fudge the numbers when it comes to how they count the graduates at five years instead of four years," said NDP Leader Andrea Horwath.
"I get concerned because sure, we went to four years, but even my own son did a victory lap. A lot of young people are taking a victory lap after Grade 12, which speaks to whether or not this was actually a successful change."
MacLeod suggested another reason for students to stay an extra year at high school: poor job prospects.
The Liberals spent hundreds of millions of dollars to increase the graduation rate, and had a team of 50 experts in the Ministry of Education working on the file, but by any calculation still fell short of their goal of an 85 per cent rate by 2010-11.
The government said the five-year rate jumped from 68 per cent in 2003 to 82 per cent last year, while the four-year rate went from 56 per cent to 73 per cent during that same time period.
Statistics Canada calculates the graduation rate by dividing the number of graduates from publicly funded high schools by the average of the population aged 17 and 18.
It says the national high school graduation rate was 74.8 per cent in 2008-09, while Ontario's was just slightly higher 75.5 per cent.
Newfoundland and Labrador lead the pack at 81.2 per cent, followed by Quebec and British Columbia at just over 80 per cent, to a low of 68.5 per cent in Nova Scotia.
Most provinces calculate the grad rate differently.
Saskatchewan reports a high school completion rate for each year ranging from students who graduate in three years to those who spent eight or more years to get a diploma. Alberta reports three- four- and five-year graduation rates.