More than 100 students are learning in portable classrooms just steps away from a brand new $8.6 million Catholic elementary school that opened in Windsor this week.

St. Teresa of Calcutta Catholic Elementary School is too small to house all of its 520 students. That's because the building was initially funded to fit only 425 based on enrolment projections in 2011.

"It's a brand new school. You would think that they would fit all the students instead of making a portable for extra students," said Aries Cabangon, who has one of his children learning inside of the portables.

To accommodate the surplus, 104 Grade 4 and Grade 5 students are doing most of their learning inside a 'Portapak' — multiple portable classrooms connected with a hallway — behind the state-of-the-art school. They must go outdoors and head into the main building to use washrooms, the gym, the library and access their lockers.

St. Teresa of Calcutta, Windsor

More than 100 students are studying in a group of portable classrooms behind the new $8.6 million school. (Jason Viau/CBC)

"We want them in [the main school] as much as possible," said principal Elise Daragon.

She admits "no one wants their child to be in a [portable classroom], obviously."

The principal added the overflow is due to a "huge influx of population" within the school's boundary since the board planned for the new building.

The school draws students from a "huge" boundary that stretches from Drouillard Road to Jefferson Boulevard and Tecumseh Road to South National Street.

When building a new school, Daragon said the Ministry of Education "tends to fund very conservatively" because the government is looking for a "stable population" of students.

Addition considered if population is stable

WECDSB spokesperson Stephen Fields said planning for new schools is "tricky business."

"We do live in an era of declining enrolment. So you could build to the exact number and if your enrolment doesn't maintain steady at those levels and it starts to drop off," said Fields. "Then you're back to square one. Then you've got half-empty buildings sitting there and your back to doing accommodation reviews and you're back to closing schools again."

St. Teresa of Calcutta, Windsor

Ministry of Education would only consider constructing an addition for the school if the larger-than-expected population proves stable. (Jason Viau/CBC)

If the school population becomes stable, the government would then consider an addition, she added.

Daragon assured parents the portable classrooms are only a temporary measure and hopes to get approval for an add-on at some point in the future.

Schools must be built for the long term

The Ministry of Education requires schools to be built for the long-term.

In an email to CBC News, spokesperson Heather Irwin said school boards are responsible for using future enrolment projections to determine if growth or decline may occur.

"New schools are usually constructed during the peak enrolment period, after which enrolment may decline," explained Irwin, adding portables may be used as a short-term solution.

The ministry encourages school boards to ensure there won't be any "unused" or "wasted" space in the future.