Pupils remembered their smart and caring friend through tears, and they left flowers, balloons, ribbons, candles and messages in a memorial along a chain-link fence outside school.
"All my friends were crying. Pretty much nothing but sadness and nobody doing anything," Christina's friend Jenae Shelby told Tucson's KGUN9-TV.
Kayley Clark, a classmate who was friends with Christina for years, told The New York Times: "I just feel shocked and very, very sad. She was very, very smart and very, very nice. She was such a fun person, and I really wish she could come back."
Christina, a third-grader, was the youngest of six people killed when a gunman began shooting at Rep. Gabrielle Giffords and her constituents outside a supermarket on Saturday. Christina, newly elected to her school's student council, was interested in government.
"She was really happy because we were both just elected to student council," best friend Jaime Stone, 10, said before tying a ribbon to the fence, according to ABC News.
On Monday, the first day back since the shooting that has shaken the nation, the school held a moment of silence and allowed students to share their memories of Christina.
"They told them it's fine to be happy when you think about Christina and it's fine to feel sad," Christine Parrish, a mother of two daughters who knew Christina, told the Times.
Eight counselors arrived to help the youngsters cope with Christina's death, and school officials were preparing for a tough week.
Crisis teams will be on hand for "many days ... as long as we need to be here," Todd Jaeger, Tucson schools associate superintendent, told ABC. "If they need help, the kids will be taken privately to rooms to receive help."
The school's staff of 18 may also need help, he said. "They care about these kids very deeply and are affected by this on a level many can't understand," he said.
Another friend, fifth-grader Rachel Cooper-Blackmore, tied a pink ribbon to the fence and left a letter to Christina.
Rachel, 10, said her mother was going to tell her about Christina's death on Sunday but she found out on her own.
"I saw her picture on the news and just started crying my eyes out," Rachel told ABC.
Classmates remembered their friend as playful, friendly, funny and caring.
"Just so sad because she was like everybody's friend. You would just walk up to her and she would just start talking to you," classmate Jordan Figueroa told KGUN9-TV.