Things seemed to be going well for Zachary Bush in the final months of kindergarten, or so his parents say they were told.

By springtime, his teacher reported, he could recite the alphabet and count past 60, and he was playing nicely with others at the private Key School. And if he still had a few little problems with coordination -- tracing, cutting and drawing -- he was zipping his own jacket and tying his own shoes with aplomb.

So when the prestigious Annapolis school announced it wanted Zachary to repeat kindergarten next fall, his parents were surprised. And then they got mad.

Now, the Mitchellville family has taken the unusual step of suing Key School -- not for money damages, but to force the school to promote their son to first grade.

School officials say they stand by the professional judgment of their teachers in the decision to hold Zachary back. But Zachary's parents, Edward J. Bush and Paula Bielski, call the school's decision "arbitrary" and "capricious." The only explanation teachers gave them, they say, was that Zachary is the youngest in his class.

"I kept asking, `What's wrong? Is there a problem with Zachary?' I could understand if there was a problem," said Bush, 41. "I don't think we ever got an answer."

The couple's lawsuit centers on two of the most emotional debates in education today: whether schools should, as many parents and politicians believe, end "social promotion" -- the practice of advancing children to stay with their peers. And whether the entry-age for kindergarten should be raised, as suggested by a growing number of teachers who complain that most children aren't ready for school before they turn 5.

In Maryland public schools, students who turn 5 by Dec. 31 can be admitted to kindergarten, although parents can request that their children wait a year.

To Bush and Bielski, their son's case is also about service. Despite paying more than $10,000 a year in tuition, they complain that the school treated them in a "bureaucratic" manner.

Key School officials would not discuss the specifics of Zachary's case, but they said they are within their rights to hold him back.

"There is a very clear principle here, and that is the school's exclusive right to make grade placement decisions in the best interests of the child, according to the professional judgment of our faculty and administration," headmaster Ronald Goldblatt said in a statement.

No court date has been scheduled.

At 5 1/2, Zachary Bush is a stocky, energetic little boy with a mischievous sparkle who knows how to multiply by 11, wants to be an FBI agent when he grows up and loves to bounce on the backyard trampoline at his family's 10-acre wooded spread in a still-rural corner of Prince George's County.

Bush, an aviation lawyer, and Bielski, vice president of an office supplies company, understand that their lawsuit may give the impression that they are driven, ambitious parents bent on keeping their offspring on a prestigious fast track.

But they insist that they would be open to holding their son back if the school gave a good reason. They said they see no shame in Zachary repeating a grade, though Bielski fears that Zachary "might feel that way."

Zachary entered Key School last fall at 4 years and 9 months of age. His sister, 7-year-old Jennifer, was a two-year veteran of the school, heading into second grade.

His parents said they were happy with Zachary's first year of school and were given no indication that he had problems. In March, they were sent a contract for his enrollment in first grade, and his father returned it with a $1,090 deposit for next year's tuition of $10,900.

The next day, they met with his teacher for a midyear conference. Despite a largely positive report card, the teacher casually suggested that they might want to consider holding Zachary back a year -- noting that with a November birthday, he was the youngest in the class.

A month and a half later, that recommendation turned into a unilateral decision by the school not to promote Zachary, his parents said.

They said that school officials told them that if they did not like the decision, they could send Zachary to another school.

Indeed, separate evaluations of Zachary found that he had some lingering difficulties, possibly related to his age.

While a report by a school administrator graded Zachary as average or above in all categories, she noted that he had a tendency to "shut down" when tasks seemed too difficult, according to court papers. While he was adept with word games and riddles, he had trouble with memorization.

Private psychologists hired by the family found that Zachary was often inattentive or impulsive, and they said his minor speech impediment might make it hard for him to link spoken sounds with written words.

Yet his parents noted that neither evaluation recommended that Zachary be held back. And they said the school never gave them a strong reason for holding him back.

"If he were having trouble with reading, and tutoring were out of the question, or if he was falling behind in math . . . I would understand that; I could see that reason," Bush said.

Bush and Bielski said they are disappointed with what they see as the school's refusal to communicate with them about their son's progress -- especially considering the amount they pay in tuition. "Your level of expectation is more than if you're in public school," Bielski said.

As for their son, they still believe that first grade is in his best interest, despite his minor difficulties.

"I'd rather have him work a little harder to keep up than twiddling his thumbs waiting for the others to catch up," Bush said. "If he's not challenged, he may be looking for other things to do."

CAPTION: Paula Bielski and Edward J. Bush are suing Key School to force the promotion of their son, Zachary Bush, 5, to first grade. Zachary's sister Jennifer, 7, attends the same school.