Then she steps through the worn, wooden doors of St. Joseph's, a Roman Catholic parish for generations of poor, Hispanic immigrants, and into a 20-seat white Hummer limo that rents for $150 an hour.
Before long, a stretch Lincoln Town Car arrives for the next Quinceañera Mass.
An elaborate coming-of-age ritual for Hispanic girls on their 15th birthday, the Quinceañera has long been divisive in the U.S. Catholic Church, where it's viewed as either an exercise in excess or a great opportunity to send a message about faith and sexual responsibility.
The latter view won an important endorsement last summer, when the Vatican approved a new set of prayers for U.S. dioceses called Bendición al cumplir quince años, or Order for the Blessing on the Fifteenth Birthday.
Consider it an acknowledgment of the changing face of American Catholicism. Latinos account for nearly 40 percent of the nation's 65 million Catholics and 71 percent of new U.S. Catholics since 1960, studies show.
Here in the Archdiocese of Denver, Latino ministry leaders view the Quinceañera craze as not just a chance to strengthen faith and family, but as a weapon against teen pregnancy.
Before Reyes could get her Quinceañera Mass, she and her parents had to enroll in a four-week curriculum introduced last year at Latino-dominated parishes that combines Catholicism 101 with a strong pro-chastity message.
``Some girls come to the class expecting to be taught how to dance,'' said Alfonso Lara, the archdiocese's Hispanic Ministry coordinator.
The girls in Reyes' class gathered in a stuffy room with a map of México on the wall and a crucifix on the table.
One lesson included tips for safe dating (avoid dating Web sites in favor of group outings in public places like the mall or family barbecues). Then there is an explanation of the difference between simple abstinence (a way to avoid pregnancy and sexually transmitted diseases), and chastity (living like Jesus and Mary).
Monica Reyes is the model pupil. Once her Quinceañera is over, the high school junior her sister calls a ``girl's girl'' will be allowed to go to parties and date, as many of her classmates do. But Reyes isn't eager to join them.
``I'm still too young,'' she said. ``I could have a bad experience. So I'd rather wait.''
In México and other Latin American countries, the Quinceañera once signaled that a girl was officially on the marriage market. The downside to that legacy: The Quinceañera Mass is sometimes seen as sexual coming-of-age moment.
Although teen pregnancy rates have generally been in decline across ethnic lines over the last 15 years, 51 percent of Latina teens get pregnant before age 20, according to the National Campaign to Prevent Teen and Unplanned Pregnancy.
``Even now, immigrant parents don't talk to their young daughters about sex,'' said Timothy Matovina, director of the Cushwa Center for the Study of American Catholicism at the University of Notre Dame. ``There is not an open conversation going on about the value of waiting till marriage or the economic pitfalls of becoming a single mother.''
Matovina said the Denver archdiocese's efforts will resonate with some families and be ignored by others, much like couples who go through the motions of marriage preparation classes to get a church wedding.
A blend of European court traditions and ceremonies from Latin American countries, the Quinceañera at times has the feel of an out-of-control prom in the United States.
A $400 million-a-year industry has sprouted up catering to Latino immigrants seeking to maintain cultural traditions while showing they've made it in their new countries, offering everything from Quinceañera planners and cruises to professional ballroom dancers to teach the ceremonial waltz.
At the same time, the ritual is a point of tension with the Catholic Church because Catholic families want their faith to be part of the celebration yet it isn't a sacrament, like marriage.
The Reyes family does not attend Mass regularly, but would never consider the Quinceañera legitimate without the blessing of a priest. A portrait of Our Lady of Guadalupe watches over the living room of the family's apartment.
``The reason to have the Mass is to be blessed, and to say thanks to God,'' said Monica's mother, Luz.
The family spared no expense, and the tension showed at times. Walking out of St. Joseph's in her gold lame dress, Luz Reyes said to no one in particular, ``Money, money, money.''
The family estimates it spent a staggering $20,000 on the Quinceañera, relying on savings, family and friends to pay for two limos, rental of a banquet hall, a buffet of Mexican and American comfort food, dresses, a DJ and more.
The cost is one reason that Monica's 14-year-old sister, Marisol, shared the church altar and dance floor with her older sister. The family couldn't fathom finding the money for another Quinceañera so soon.
Lara, of the Denver Archdiocese, said one goal of the classes is to send the message that it's all right to arrive at church in a minivan instead of a Hummer—unless there's plenty of money to send the girl to college, too.
The expense is worth it to the Reyes family, even if only now they will begin saving for college.
``It's a prize for them being good,'' Luz Reyes said.
It's also the U.S.-American dream realized. Reyes is giving her daughters something she never got growing up in Ciudad Juárez, Mexico, where her Quinceañera dress was a tattered gown and dessert was a simple layer cake.
There were perhaps 15 people at the Reyes' Quinceañera Mass. The rental hall, Martha's Golden Palace, has a capacity of 500, and Monica welcomed most of her classmates, a favorite teacher and the police officer assigned to her high school.
After an hour, the DJ turned down the deafening border music and strobe lights, and played the waltz that Monica and her court had been practicing for weeks in her apartment complex parking lot.
Later, Monica wiped away tears as she danced with her grandfather.
On the dance floor, she changed from flat shoes into heels, signaling her departure from childhood.
Her first meal as a woman was a bowl of beans washed down with strawberry soda.
``The big thing isn't to have a party,'' Monica said. ``It's that you're going from a little girl to a woman. You're thanking God you have been in this world for 15 years.''