SOUTH PORTLAND — The holiday break ended Friday morning for the staff of South Portland High School, but no one seemed upset about being back at work.

After 33 months of construction at the school, the faculty appeared to heave a collective sigh – followed by a lot of smiling – as they prepared never-before-used classrooms for Monday, when the entire new and improved building opens to students for the first time.

“People are excited,” said Principal Ryan Caron, as he walked the halls helping teachers settle into what he called their “forever homes.” Throughout the construction, which began in 2012, some have had to move classrooms four times.

“They’ve been through a lot,” said Superintendent Suzanne Godin, who spent Friday making sure all the furniture was in the right place, while doling out hugs and wishing the staff happy New Year.

A year ago, a 100,000-square-foot addition opened to the high school’s 900 students. Since then, that’s where all learning has taken place. Teachers shared classrooms, rotating in and out between periods, and turned corners of the building into makeshift learning centers.

“It was tight,” Godin said.

On Sunday, a grand opening will celebrate the completion of an overhaul to the 209,000-square-foot original building – and the $47.3 million project as a whole – with a ribbon cutting at 2 p.m. that will be followed by an open house for the community. After all, that’s who footed the bill.

Because of the city’s strong tax base and the lack of crowding at the high school, the project probably wouldn’t have qualified for state aid, and officials didn’t want to go through the lengthy process of applying for it because of concerns about the school’s accreditation status. In 2009, the New England Association of Schools and Colleges put the high school on warning status because of problems with the 62-year-old building, which did not comply with Americans with Disabilities Act criteria, and had serious structural and air-quality issues.

Initially, residents rejected a proposal to renovate the school. The $56 million plan that voters shot down in 2007 included a second gym and artificial turf field, features that were later scrapped. In 2010, voters agreed to borrow $41.5 million for the project. The rest of the funding is coming from a facilities reserve account, surplus funds and energy performance credits.

The upgrade is among the highest-price high school projects in state history, and is believed to be the most expensive one paid for entirely without state funds, according to the Maine Department of Education.

At the time of the vote, the added tax burden for a house valued at $200,000 was estimated to be $218 in 2015, the peak year, and the total impact over the 20-year life of the bond would be $3,646 for that household.

The renovated section opening Sunday includes a hallway connecting to the year-old addition, making it easier for students to get from one end of the building to the other – and hopefully lessening their tendency to cut through the courtyard that tractor-trailers use to make deliveries.

Students also will be safer now that there’s a centralized security system that can lock all exits and entrances. Previously, anyone could walk out and leave one of the 22 doors unlocked.

Other upgrades include windows that open, a more efficient heating system and enough electrical outlets to handle high-tech teaching tools, including the iPads that connect to the new smartboards in every classroom and are issued to every student.

Although Latin and social studies teacher Tom Major is looking forward to trying out the new technology, he’s excited about a more basic improvement — that the walls connect to the floor. Shifting in the school’s foundation had caused them to separate and tilt.

“The fact that this is structurally sound is a huge advantage,” said Major, who had his collection of finger puppets hanging on the wall of his new room before 10 a.m. Friday.

Throughout the building, boxes sat on desks and rolled up posters waited to be unfurled. Meanwhile, construction workers made finishing touches that needed to be taken care of – a missing floor tile here, a cabinet to install there. But aside from getting everyone used to the new drop-off traffic pattern, Caron said, there was nothing left to prevent a smooth opening.

“I’m glad we’re getting to the end of it,” he said.

Rick Romanow, a history teacher, had country music playing as he finished decorating his room Friday morning, while noting that the most important piece was yet to come.

“Just having the kids in here,” he said. “It’s their school and they’ve got a lot of pride in it.”