Students attending a Catholic high school in the GTA are graduating at a significantly higher rate than those attending a public school, according to new numbers released by the province Tuesday.

Education minister Liz Sandals announced the high school graduation rate in Ontario continued to increase in the 2014-2015 school year, with 78.3 per cent of students finishing high school in four years with an Ontario Secondary School Diploma, or OSSD.

That's up from 76.3 per cent in 2013-2014.

Ontario also continues to publish the number of students who finish high school in five years, even though it's officially been a four-year program since 2003.

When those five-year students are included, the rate increases to 85.5 per cent.

Big differences board by board

For just the second time, the education ministry also released the graduation rates for all 72 school boards in the province.

"We want to ensure that parents, students, teachers, school boards and education workers have access to the local results that can help inform efforts to improve those grad rates even further," said Sandals.

Those local results expose a trend where the smallest boards consistently have the greatest success at graduating students.

CBC News broke down the rates by the province's four publicly-funded boards — English public, Catholic, French, and French Catholic — and found a gap in the number of GTA students who finish high school in four years.

The English public boards — where two-thirds of students attend — have the lowest graduation rate.

The French Catholic board — home to just 1% of students — has the highest.

*Conseil Scolaire Viamonde includes high schools in GTA, southwestern Ontario, Niagara Region, and north to Penetanguishene

**Conseil scolaire de district Catholique Centre-Sud includes high schools in GTA, Hamilton, Cambridge, Barrie and Welland

Kenneth Leithwood, a University of Toronto professor who specializes in educational policy, says differences in graduation rates across the public, Catholic and French boards have puzzled educators for years.

"Some believe that Catholic schools often feel that their continued existence is threatened so they focus more on their students' success," he says. "Some suggest that the shared set of values supported by Catholic schools contributes to a more unified school culture."

Leithwood adds another theory — that public schools admit a more diverse student population including more kids with special needs than Catholic and French schools.

Annie Kidder, executive director of the independent advocacy group People for Education, says one reason for the Catholic boards' success might be that parents who choose to send their child to a Catholic school are already making a conscious decision to be engaged in their child's education.

*Conseil Scolaire Viamonde includes high schools in GTA, southwestern Ontario, Niagara Region, and north to Penetanguishene

**Conseil scolaire de district Catholique Centre-Sud includes high schools in GTA, Hamilton, Cambridge, Barrie and Welland

"Not necessarily because one is a great education system and one is not," she says. "There's no evidence of that."

Catholic schools also typically have better results on EQAO assessments, the province-wide reading and math tests, according to EQAO CEO Bruce Rodrigues. But Rodrigues, a former director with the Toronto District Catholic School Board, cautions against looking for simple answers to explain differences in graduation rates.

When CBC News asked the education minister for a possible explanation, she had little to offer, except that demographics might be a factor.

Sandals did acknowledge the inconsistencies in graduation rates across the province: "We know that there are some boards that we would like to see the graduation rates go up. But we know that's also a matter of pursuing one of our other goals, which is the goal of narrowing achievement gaps."

TDSB lags behind other GTA boards

When the rates are broken by geography, numbers show the Toronto District School Board (TDSB) lags behind its suburban counterparts, with a graduation rate of 71.6%

York Region Catholic board has the highest graduation rates, at 93.2%

TDSB has its own method of calculating graduation rates, which it says accounts for the high number of students who join or leave the board in the middle of their high school career.

TDSB research co-ordinator Rob Brown says about 10% of students will transfer out of high school to another board or another country.

According to TDSB's numbers, the board's five-year graduation rate has increased 5% since 2012.

Brown says though most students in academic programs finish high school in four years, those not headed to university often stay for a fifth.

Programs to increase grad rates

The province launched its $3.3 billion Student Success Strategy in 2003 to boost graduation rates, and offer struggling kids alternatives to the traditional academic credits needed for an OSSD.

Most focus on hands-on training that leads to a specific career path and include co-op placements and career studies.

Tuesday morning at Danforth Collegiate and Technical Institute in Toronto grade 10 students were whipping up lasagnas as part of the school's hospitality program. The program gives students a certificate that lets them easily move on to apprenticeship or college programs.

Another program lets students get high school and college credits at the same time. There are also many special education programs in place, and the school has a social worker on staff.

Aleah Persaud came to Toronto from Guyana six years ago and credits the school's extra supports for pushing her to graduate. She plans to start Seneca College's biotechnology program in the fall.

"The students and the teachers they have a very unique relationship. They're not based on just academics, and do your work. They all bond. They all have a special relationship with each and every student."

The TDSB says programs like these helped improve the school's 2015 graduation rate by 10% over the previous year -- the biggest jump of any school in the board.