Mr Benson said the visit was great for the pupils, as many of the cricket fans were "in admiration of his achievements".
"Every lunchtime the kids are out playing cricket," he said.
Williamson, who was swarmed by youthful fans for autographs, got in a quick game with the pupils after officially opening the pitch.
"It's cool, great to see the kids playing cricket," Williamson said.
After Williamson cut the black-and-yellow hazard tape, the pupils ran onto the pitch and some jumped over the wickets in excitement.
Ten-year-old cricketer Richard Denyer, who was fielding the game, said he thought Williamson "was really cool".
Te Manewha Rikihana, 9, said he and his friends had been waiting in anticipation for Williamson to arrive.
"We'd been holding out since lunchtime," Te Manewha said. "It's pretty cool and very exciting."
Mount Maunganui Primary School Principal Damien Harris said the school had been looking at developing the cricket pitch for a couple of years, and having Williamson open it was a great opportunity for the pupils.
"The chance to get him down here is just awesome," Mr Harris said.
Williamson said he would be in the Bay of Plenty until Thursday and is planning on getting some time to relax before he leaves.
Earlier, he visited his old school, Tauranga Boys' College.