Last year St Gregory's College welcomed female students for the first time.
Now the new junior school will offer even more places for girls and boys from kindergarten to year 6.
Head of St Gregory's College junior school Joel Weekes said it was “exciting” times for all involved.
'We will be going from one stream to two streams," he said.
"We've had a strong first year and I think having two classes of each grade will give the kids more exposure to different social situations, sporting opportunities and more."
Eight new classrooms and the junior school library will be built this year in time for new students to walk through the gates in 2020.
Mr Weekes – who attended school down the road at Mount Carmel Catholic College, Varroville – was appointed as the head of junior school in 2016.
Since the appointment, he spent every day ensuring the school ran without a hitch.
Mr Weekes said the co-education model had been going well.
"We've been very boy-heavy in the upper years but it has been really positive," he said.
"The handful of girls in year six have really banded together well and they seem to get along with the boys too.
"It has been a really positive move and the school has a nice feel to it."
Mr Weekes said could not confirm if the school would expand any further after stage two works were complete.
"It will all depend on enrolments and finance," he said.
"We're hopeful for a strong enrolment this year."
The college is currently enroling now for 2020.
The junior school is currently taking enrollments for kindergarten and for years 3 to 6.
The high school was established about 100 years ago and caters for male students only.
To enrol, collect an enrolment pack from the junior school or contact Daniela Romeo on 4624 2700.