Sneaky, secretive behavior by the students at Sylvan Elementary School sure made their principal happy Tuesday. For about six weeks, the 373 kids have found opportunities to practice a dance choreographed to the Pharrell Williams hit song “Happy.” Most of the rehearsal was class by class, behind closed doors. On the few occasions when Principal Venessa Lind was off-campus, the whole student body got together. The culmination of all that practice was a flash-mob surprise to show appreciation for Lind. “I was overwhelmed,” the principal said Tuesday. The flash mob had been timed for the end of recess, when all the children stop where they are and take a knee when the bell rings. Lind had been meeting with a district administrator, who was in on the secret. She urged Lind to go outside as the bell was set to ring; it did, but the music that was supposed to kick in was delayed by a technical glitch. “It was a minute past recess, and I’m thinking, ‘Why are they still frozen?’ ” said Lind, who is in her fourth year at Sylvan. “Oh, my goodness, I have my superintendent here and they’re still frozen.” Glancing around, she saw a couple of other district officials on campus, and it occurred to her, “Something’s up here.” Then the music kicked in and her tribute began.