The aging, blue Bible Bus rolls into the driveway of the C.M. Bradley public school every Friday morning and, for the rest of the day, the fourth graders hop aboard to praise the Lord with Mary Prince.

On the bus, a makeshift classroom with painted stained-glass windows, Prince reads to her students from the scriptures and sends them off chanting songs such as "The Lord's Army" and "Jesus Loves Me."

"The Bible is our guide to daily living," she tells them. "If we listen to what God says, we'll be OK."

From here in Fauquier County on the fringes of the Washington suburbs to Bristol on the Tennessee border, thousands of Virginia public school students are still attending such classes, filled with Christian prayers and hymns. Although enrollment is voluntary -- parents give written permission for their children to attend -- the existence of Bible classes on school property during school hours has rekindled a constitutional debate over the role of religion in the public classroom.

In recent weeks, it is a battle that has erupted throughout the state, pitting civil liberties groups against Bible-quoting religious fundamentalists.

"These classes amount to free room and board and essentially a captive population for religious indoctrination," says Judy Goldberg, a lobbyist for the Virginia chapter of the American Civil Liberties Union, which has been spearheading a campaign against the programs. "They give the stamp of state approval for particular religious beliefs."

Since the ACLU and the state Anti-Defamation League began raising the issue earlier this year, they have identified 28 school districts in which Bible classes and other practices, including prayer and religious assemblies, occur with varying degrees of regularity.

In one southwestern high school, the ACLU discovered, prayers were being recited over the intercoms and religious signs -- "He Is Risen" -- hung in the hallways. In a Richmond suburb, teachers distributed Gideon Bibles to pupils and conducted religious assemblies where evangelical Christians proselytized for Christ.

The persistence of such practices, despite numerous Supreme Court rulings forbidding them, suggests the power of tradition in Virginia's largely Baptist Bible belt. Yet it is a tradition that until recently has been virtually ignored by state officials in Richmond and sanctioned by the mainstream Virginia Council of Churches, the largest ecumenical group in the state.

Since 1929, the council or its predecessor has run a Weekday Religious Education program that furnishes biblical literature and employs a part-time coordinator to supervise locally operated Bible classes around the state. The council's "consultive" services have been offered to the local groups, regardless of whether the classes were conducted on school grounds, although a mildly worded resolution passed last month urges that the services be brought "within constitutional guidelines" by July 1, 1984.

Meanwhile, state education officials, who recently drafted a similarly worded directive to local school boards, say they were only recently aware of the problem.

"You've got to understand Virginia," said W.H. Cochran, deputy superintendent of education. "In many parts of the state, this kind of thing can be going on and the public thinks it's just fine. Until this time, I don't know that any parents had even been making an issue of this."

The civil liberties groups, which criticize the state for lax supervision, are unimpressed. "There's no dispute that these practices are unconstitutional," says Norman Olshansky, regional director of the state Anti-Defamation League. "But people have been afraid, and understandably so, to raise the issue and question them. I can't tell you how many times we've gotten calls from parents who are concerned about this but don't want their names used and were afraid of any publicity.

"It's sort of like questioning apple pie and motherhood," he says.

The new campaign, though, is already having an impact. At least three school districts, under the threat of ACLU lawsuits, have recently decreed an end to Bible classes, although not without facing political heat.

When the school board in Alleghany County voted last month to discontinue its weekday religious program, more than 200 parents and preachers, many bearing Bibles and quoting scriptures, crowded into a small stuffy board room to voice their protests.

And when the superintendent of Danville's public schools, concerned about evangelizing at a school assembly, ordered principals to forbid such events, he was flooded with telephone calls and letters from angry parents. One local pastor denounced him for succumbing to "progressive humanism" that he said was moving the schools "one step closer" to those in the Soviet Union.

The atmosphere here in Fauquier, a rolling stretch of farmland about 55 miles southwest of Washington, has been considerably more subdued. For 44 years, with barely a murmur of protest, a group called the county Weekday Religious Education Council has been offering weekly Bible class to public school students.

During the early 1970s there were concerns raised about such classes in school buildings so the council purchased a school bus and converted it into what was dubbed the "Bible Bus." Now, the energetic Mary Prince makes the rounds of the county's six elementary schools, parking in the driveways and then leading the students single file onto the Bible Bus for her 40-minute classes.

Her arrival appears to be popular among most students, who were enthusiastic during her dramatic rendition of the David and Goliath story in a class last week at the Bradley school.

"Fourth graders look forward to anything that's different," noted Michael Helms, a teacher at the school. "They greet the PE teacher with cheers. They greet the arts teacher with cheers . . . They greet Mrs. Prince the same way."

This year, 90.8 percent of the county's 431 fourth grade pupils attend the Bible classes taught by Prince, an employe of the religious council. Those that don't attend -- usually two or three per class -- stay behind in the classroom and "read a library book, do their arithmetic or whatever else they need to do," says S. Harold Lamm, the school superintendent.

"There's been almost no objection to this in Fauquier County," Lamm adds. "I think there's been only one question raised about it in the last 10 years . . . Why don't you write about sex education? There's been a lot more questions about that."

If Lamm and other school officials are surprised by the controversy, some parents aren't. Lana Hankinson of Warrenton chose not to send her two sons, "because we felt that they received enough religion in our Catholic catechism on Sundays after Mass.

"I felt that class had a Baptist slant," she said. "I didn't want them being confused and mixed up with other religions at their age . . . I don't think they should be taking up school time with this."

But like several other parents interviewed, all of whom declined to be identified publicly, Hankinson was faced with subtle pressures from the schools to have her children conform.

When she first withheld permission for her older son, Michael, to attend, his public school teacher peppered her with notes saying "it would be good for him." And she recalls that her younger son, Christopher, would come home after school and "want to know why he couldn't go. He sort of felt that he was being left out."

It is such pressure that is, in the end, the worst aspect of the Bible classes, says Anti-Defamation director Olshansky. "It's putting children in the position of being caught between their religious convictions and the approval of their teachers and peers," he says. "And that's something that should not be happening in public education."

If there is a legal challenge to the Fauquier County Bible Bus -- and at least one Warrenton parent says she may bring one -- it is likely to turn on a narrow legal issue: whether the driveways where the bus parks are on school property. Superintendent Lamm, in written responses to the parent last February, acknowledged that they are, an admission that may make the program unconstitutional under Supreme Court rulings going back to 1948.

Last week, however, Lamm said in an interview that the driveways weren't school land, because "they are public numbered drives maintained by the state of Virginia."

Either way, Bible teacher Mary Prince says she doesn't see how it makes any difference. "We talk about the land," she says. "But who does the land belong to really? It belongs to the Lord."