Joe Negron, the founding principal of KIPP Infinity Middle School, picked a tricky year to return full-time to the classroom. After heading the charter school since it opened in 2005, Negron became a full-time math teacher in August, just as new standards were reshaping what students are supposed to learn.

Negron taught math at I.S.164 before starting KIPP Infinity and kept one foot in the classroom while serving as principal. But even for a seasoned educator, he said, shifting to the Common Core standards is a challenge.

"This is so much messier than what I'm used to and comfortable with," said Negron, who will appear this evening on the panel at GothamSchools' event about the Common Core in math. "It used to be, I'm going to teach you this strategy and you're going to use it until your eyes pop out."

This year, responding to the Common Core's emphasis on solving problems in multiple ways, he is asking students to master three approaches.

The old strategy, Negron said, was more immediately satisfying to teachers and students: Students could feel accomplished more quickly and teachers could more easily measure understanding.

But the old way did not guarantee that students actually grasped the underlying mathematical concepts, Negron said. "The way we've done it has created some pretty good robots," he said.

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Now, he is pushing students to tackle the same problem with several different strategies, and identify which approach might be most appropriate in a given situation. Developing that kind of flexibility in students and teachers alike, Negron said, requires "a mind shift."

Here's an assignment that Negron and his colleague Angela Fascilla used to get their fifth-graders thinking about different ways to multiply fractions. The first of eight Standards for Mathematical Practice, which together describe the skills students of all ages should develop as they master grade-level content, emphasize the importance of this skill.

"Mathematically proficient students check their answers to problems using a different method, and they continually ask themselves, 'Does this make sense?'" the standard reads. "They can understand the approaches of others to solving complex problems and identify correspondences between different approaches."

These examples of student work will be on display at our event tonight, along with student work from other teachers who are participating in the discussion.