The ceramic tiles are being packed in boxes, carefully stacked between sheets of tissue, for their ride to Cheverly and an eight-hour bake in an electric kiln, when Tom O'Reilly spies a problem.

"Is this tile 156 or 159?" the puzzled parent asks.

O'Reilly is one of dozens of volunteers who have spent countless days helping students at Arlington's Glebe Elementary School assemble, paint and pack 2,700 hand-glazed tiles. Next fall, the six-inch-square pieces will be put together to form a two-story mosaic mural dubbed "The Big Picture" and installed inside the newly remodeled school on Glebe Road.

Officials describe the bold, permanent art installation as one of the most ambitious art project ever undertaken by Arlington County public school students.

Nearly all of Glebe's 300 or so students -- pre-kindergarten through fifth grade, plus special education -- have participated in the project, first by submitting drawings that were melded together into a cohesive design by art teacher Stacey Lewis and then tackling the weeks-long process of glazing.

With a project this size, every move is choreographed and mapped.

Having spent seven weeks working on the mural, O'Reilly knew just what to do with the mystery tile he had found. First, he checked it against a gigantic color-coded master list affixed to the classroom wall. Then he compared his findings to a scaled schematic depicting each individual piece of the mosaic.

In his hand, he decided, was either a piece of panda or a swath of green background. After a quick consult with Lewis, the errant tile was renumbered (its number had been wiped off during the cleaning process) and stored in a box with its terracotta brethren.

"This is exactly what can happen," explained expert ceramist Alfredo Ratinoff, a paid consultant on the project whose clients include notables from Hollywood and national corporations. He has 37 murals to his credit, several of them at schools.

These days, the native Argentinean has been knee-deep in grade-schoolers, showing them the correct way to apply glaze and laughing at the inevitable slips. He will spend the summer baking the tiles. His kiln can hold only 60 tiles at a time. Its temperature rises to 1,891 degrees and has to be watched "like a baby," he said. Later a professional company will be brought in to install the mural, which, Ratinoff says, will suffer the inevitable breaks and cracks.

"Broken tiles are a good" omen, he said, beaming, his fingers white with a low-fire opaque glaze.

The mural was born from a need to decorate a large wall in the renovated school building, which is expected to open late fall or early winter. Students have been attending school in an alternative facility in Rosslyn.

The final cost of the mural has not been calculated,but school officials estimate it will cost at least $21,000 -- including $9,000 for installation, $3,000 for firing, $5,000 for Ratinoff's expertise and $4,000 for the tiles and glazes.

Officials said about $12,500 was donated by parents and local businesses, with the county contributing $7,500 and the PTA about $2,500.

"The response from the parents has been astonishing," said PTA member Brooks Belford, who headed the fundraising team. "It's probably because the whole school has gotten so excited about it. It's so cool for a parent to think their kid is creating this piece of art . . . that will be a part of the school for a generation."

Because the mural likely will hang for decades, Lewis was careful not to depict themes that could become dated. She did, however, bow to the students' desire to illustrate a cicada. It was a last-minute addition.

"The wall was screaming for something artistic. Something special," said Glebe Principal Sylvia Taub. "We talked about a lot of things, but this was the undertaking that was decided on."

Lewis admits that she had no idea how much work would be involved in making the mural, which will be 26 feet high and 28 feet wide. Hoping to gain some knowledge, she and arts committee chairwoman Lynn Westergren took a class taught by Ratinoff at Alexandria's Torpedo Factory. He signed on to help. But even then, she still had only a vague idea about the magnitude of the project.

Ratinoff's blueprint for the mural set her straight.

"He said, 'You're going to die when you see how big the mural will be,' " Lewis recalled. "When we saw it, we screamed."

But parents flocked to volunteer, and students were eager to participate, submitting more than 300 drawings for consideration. The mural's design was broken down into six sections, each containing elements offered by the students. Lewis drew the mural, but the animals and plants she used came straight from the children's drawings. One look at the toucan drawing submitted by Charles Massiah, 12, and you can see its near-identical reproduction in the mural.

"They can look at it and say, 'That came from me,' " Lewis said.

"It was a group effort," echoed Westergren. "It was everybody's baby."

Last week, students were winding up the glazing process. The mural had easily overtaken the small art room, where gallon bottles of glaze sat propped against the sink, dozens of boxes were taped up and ready to be driven to Ratinoff's Cheverly studio, and tables were covered in tiles ready for the last round of glazing.

Rather than use paintbrushes, students applied the glaze using small rubber syringes designed to put drops in a baby's ears but which let the students squirt color onto the tiles in one thick coat. The kindergarteners, who designed and worked on the lower half of the mural, which depicts flowers and underwater life, used a sponging technique that, after firing in a kiln, will create a mottled effect.

Fifth-grader Andrew Wisenberg, 10, was responsible for the mural's "Egyptian sharp-winged hawk," a bird he created from his imagination that soon will adorn his school's atrium.

Last week, he came in for his third chance to paint tiles, smearing them with a glaze that, once applied, looks a lot like fondant icing.

"You think this is cool?" a visitor asked.

Andrew nodded knowingly. "Yeah," he said. "Definitely."

Under the watchful eye of ceramist and consultant Alfredo Ratinoff, Glebe Elementary students Nick Hallowell, 11, left, Erin Hogans, 11, and Cynthia Amerson, 11, add color to "The Big Picture."Art teacher Stacey Lewis melded hundreds of drawings into the design.A print of the mural illustrates the myriad contributions: "It was a group effort," said one participant.A bird created by student Katie Labadie, 11, found a home on the mural.Finished tiles await installation as part of the two-story mosaic mural that will adorn a wall in the renovated Glebe Elementary School, expected to reopen sometime in the fall.