On Saturdays at Rancho Cucamonga High School, the indoor-outdoor campus can resemble a community carnival.

Last week at 7:30 a.m., teams of spandex-clad, color-guard flag bearers warmed up for a competition on whatever patch of concrete was available. Setup was beginning for a Black cultural celebration.

And on a lower level, in a classroom sandwiched by a “Cash for College” meeting and a study session for students in A.P. Chinese, a group of teenagers filled out tax returns for anyone who had heard about their free clinic.

Three generations of one family came, having heard about the clinic from the fourth — the great-grandmother. They brought along a brand-new dependent: a 2-month-old baby. A boy in a Spider-Man suit watched Spider-Man videos while his grandparents received help. Two retirees — one a Harley enthusiast and another who arrived on a spiffy electric bicycle — checked in as well.

That anyone would trust high school students to prepare their returns is the doing of a little-known service called VITA, which stands for Volunteer Income Tax Assistance. It is an Internal Revenue Service program that trains people to help their neighbors with the annual task.

To participate, the students must plow through a kind of Income Tax 101 curriculum and then take tests drawn from Form 6744. (The I.R.S. has a form or publication for basically everything.)