The band-rehearsal room at Seguin High School was vibrating with adolescent jitters. While one member of Mariachi Aguilas struggled to tune his guitarrón, director Kevin Elizalde patiently coached him through. The teen’s bandmates, standing at quiet attention in their ornate gold-and-blue trajes de charro, were getting antsier with each passing second. “Hurry it up,” snapped a petite vihuela player, finally breaking the silent tension and eliciting a burst of laughter from her bandmates and their small cadre of chaperones. The guitarrón player, also chuckling, found his pitch, and the band ran through its two numbers. The students seemed more relaxed after having warmed up, but this was still their first trip to the UIL State Mariachi Festival, and they’d traveled more than seven hours, from San…