You know that feeling when you open a bag of chips, eat a few more than you'd planned, and think, "Oh well, I might as well finish it?"

Well, that's kind of what happened to 18-year-old Phillip Sossou of Boston, Massachusetts. Only instead of eating chips, he was drawing charcoal portraits — of his entire senior class.

"My teacher told me I was fast at drawing," Sossou told TODAY. "So I decided to put that to the test."

Sossou had registered for an AP visual arts class at Boston Latin School without having taken any department prerequisites.

After getting positive references from other faculty members, instructor Stephen Harris allowed Sossou to enroll — but insisted he put in some extra work to make up for the lack of prior instruction.

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"Students in the advanced classes normally have at least 20 pieces of work in their portfolios. Phillip had zero," Harris told TODAY. "We came up with a schedule of things he could do ... the first project was a charcoal self-portrait. I showed him how to work with the material, and he loved it."

It was that first exercise that sparked Sossou's idea to begin sketching his classmates. After obtaining a list of students from the registrar, he began working his way through all 411 seniors.

It's often said that it takes 30 days to make a habit — and sure enough, once Sossou had made a good dent in the project, he didn't want to stop.

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"I would have felt bad if someone accidentally got left out," he said.

While Sossou didn't know every student personally, he looked at photos for guidance, pulling them from Facebook or snapping them with his camera phone on the sly.

By February, though, he had only completed a handful of portraits.

"We sat down and did the math, and realized he would have to do 16 a day to finish," said Harris. "Even very skilled students only get five or six a day, tops."

But Sossou was determined. He sketched every day for four months, sometimes staying at school until 4:30 or 5 p.m. — then continuing to work from home well into the evening.

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"It happened slowly," Harris recalled. "I give every student a folder to keep their work in, and he filled that up quickly. So I gave him a special drawer. Then he filled that up, so I gave him a special box."

The finished portraits are detailed and evocative, and their sheer number is impressive. But Sossou's grand finale was even more stunning.

Last Friday morning, Sossou snuck into Boston Latin School at 6:30 a.m. to hang the portraits around the school — a surprise to all but his closest friends.

"Parents were crying. Students were crying," he recalled. "Since I was working on it for so long, I became desensitized. But yeah, I guess it was pretty cool."

The other 410 seniors will get to keep their portraits. And while the whole thing began as a personal challenge, Sossou is thrilled to have made his fellow students feel seen, and to have left his mark on the Boston Latin community.

"It's an amazing school with amazing people," he said.

And after spending all year as his mentor, Harris had plenty of praise for Sossou as well.

"Phil is very intellectual," he said. "Very open and funny. I'll be doing something with another student, and next thing I know he's leaning over my arm to see what we're talking about, saying, 'I just want to learn.'"