Two Rivers students cheer on D.C. Teacher of the Year Bill Day, who received a $5,000 check from Mayor Vincent C. Gray. (Emma Brown / The Washington Post)

D.C. Mayor Vincent C. Gray stopped into an all-school assembly at Two Rivers Public Charter School on Friday morning to surprise math teacher Bill Day with the news that he has been selected as the District’s 2013-14 teacher of the year.

Hundreds of Two River students erupted in cheers for Mr. Bill, as he is known, as Gray (D) presented a $5,000 check and a heap of praise, calling Day “everything that a teacher should be.”

“Wow,” said Day, praising his fellow teachers as brilliant and creative thinkers and his students for their energy and hard work. “Every time you come into class ready to learn, every time you come into class ready to learn,” he said, “that’s what inspires me to work as hard as I have.”

Day started teaching more than a decade ago, and since 2011 has taught middle-schoolers at Two Rivers in Northeast Washington, a high-performing school rooted in experiential learning. Day is helping lead a “mathematical revolution” by using real-life problems to engage students in math, said Executive Director Jessica Wodatch.

His students recently learned the concept of unit rates by putting on a grilled-cheese sandwich sale for their fellow students — a task that involved all kinds of math, from calculating the cost of organic and non-organic ingredients to figuring out how much they should charge to ensure a profit.

“You want them to see that we do math to solve problems, not that we have problems so we can do math,” Day said.

Madison Williams, an eighth-grader at Two Rivers who was in Day’s math class two years ago, said she was proud of her teacher.

“There were some times he gave me hard problems ... but he helped me work through those times, he made me more confident with math,” Williams said. “Mr. Bill, he puts a lot of effort into his kids, so it makes learning more interesting.”

A panel of teachers, principals and other education experts chose Day from nominees that included teachers in both traditional and charter schools. Finalists for the award included Laurel Horn of Thurgood Marshall Academy and William Taylor of Wheatley Education Campus.