Walker Kroubalkian said he didn’t pay much mind to an advance placement test he’d taken five months ago.

The University High School student took the AP calculus exam in May. He’d hoped for a 5, which is the highest composite score a student can get, but did not know how he fared until last week.

So when he was told he earned a perfect score — in other words, receiving every point possible on the difficult college-credit test — it just felt “weird,” he said. “I never thought that it would actually happen.”

Many students earn 5s on the AP tests, but very few do it with perfect scores.

Kroubalkian is one of 11 students around the world this year to have achieved a perfect score on the College Board’s AP Calculus BC exam, which is equivalent to a first-semester college calculus course. He was just a freshman when he took the test, which mostly seniors take.

About 30 percent of UHS seniors complete the AP calculus course by the end of senior year on average, said Leiba Schuneman, the math department chair. In the last 14 years, Schuneman has seen only two freshmen test directly into AP Calculus BC. Kroubalkian, now a sophomore, is one of those two.

“I’ve never seen a perfect score before,” said Barry Callesen, a UHS math teacher and math club sponsor who has taught for more than 30 years. He’s had plenty of brilliant math students in his time, but no one quite like Kroubalkian.

“I don’t think I’ve ever had someone who has the combination that Walker has.”

That combination, he said, is talent and drive.

The sophomore’s talent in math was recognized early on by his teachers and parents. Callesen noticed him when he came to math competitions at UHS as a sixth-grader.

Two years later, when some students on the UHS math team got sick and couldn’t make it to a tournament, Callesen knew who to call to fill the spot: Kroubalkian, who was in eighth grade at the time.

“Then he beat all my kids,” the teacher said.

Now, the sophomore is somewhat of a fixture in the math club, which has more than 90 students, Callesen said. Not only does he win a bunch of trophies, he also helps other students learn. Kroubalkian is also coaching the math club at his old middle school, Sonoran Science Academy.

Kroubalkian’s parents, John and Denise Kroubalkian, say they don’t really force studying on him. He isn’t enrolled in any special math programs or forced to take up more than he wants to.

“It has to be from within,” Denise said. “The passion has to be there.”

They merely support him in what he wants to do, such as attending free tutoring sessions at the University of Arizona every Thursday, participating in math club activities at school, which involves morning practices, and using web resources to challenge himself.

“It comes easy to him, so honestly, we’ve never had to give a lot of support,” Denise Kroubalkian said.

The sophomore said struggling through a problem is what really helps him learn. He also likes the strategies involved in complex mathematical problem-solving and the feeling he gets when he finally solves something. “He twinkles” when he does that, Callesen said.

He spends about two hours a day, at most, searching for challenging problems or practice tests on artofproblemsolving.com, an educational website that provides resources for various levels of math and programming, he said. He is constantly searching for the next topic to tackle, whether it’s differential equations, programming or cryptography.

It can be frustrating to find ways to challenge himself; after all, the highest level of math offered at UHS is the AP Calculus BC course he’s already conquered. But the teen knows mathematics is a huge and always growing field. He will continue to pursue solving problems.

“I do math because I want to do math,” he said.

Contact reporter Yoohyun Jung at 573-4243 or yjung@tucson.com. On Twitter: @yoohyun_jung