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SAN ANTONIO, Texas — It is the sound of Mexico, often heard in cantinas, or bars and restaurants — Mariachi.

This form of folk music has traditionally been played exclusively by men, but that may be changing. In a major shift in recent years, more and more women are beginning to play mariachi — especially in John Nieto’s San Antonio high school classroom.

“It's been a steady increase, not all of a sudden,” says Nieto, Mariachi Program Director for the San Antonio Independent School District. “The level of proficiency in their instruments has really grown too, so it's not just 'oh, they're girls.’ It's like, 'wow, they're girls.'”

Of the 129 students in his Brackenridge High School classroom, more than half are girls.

This wasn’t always the case. In the 1970s, mariachi instruction was just an idea for the now 85-year-old Belle Ortiz, known to many in San Antonio as the “Godmother of Mariachi.”

“I fell in love with mariachi music at a very early age,” Ortiz tells NBC News.

At age six, Ortiz visited Guadalajara, Mexico with her grandparents and immediately felt connected to the sounds of her culture. She learned to play the piano and was fascinated by the rhythms and notes performed by the mariachis.

Wanting to create the same music in her hometown of San Antonio, Ortiz pursued a teaching career with the hopes of bringing mariachi music instruction and historical education of the genre into the classroom.

“I compared it to any form of music, whether its band, choir, orchestra, or jazz,” she said. “The first thing that we have to do is to learn the music. Not just by ear, but with notes.”