Faithful Catholics, John and Catherine Duggan will reluctantly transfer the youngest of their seven children to a public school next month after educating all of his brothers and sisters in parochial schools.

The Duggans and at least two dozen of their fellow parishioners at St. Mary's Church, Piscataway, in lower Prince George's County, are pulling their children out of the 200-pupil school because, they contend, continuing turmoil has made any education - let alone a Catholic one - impossible.

St. Mary's, the largest parish in area in the Washington archdiocese, has become a battleground. On one side of this struggle are the old-style, priest-centered, father-knowsbest catholics who have known the parish since it was almost all covered by tobacco fields. On the other side are the newly arrived modern Catholic lay men and women, who want more of a voice in parish affairs.

St. Mary's is, to some degree, a microcosm of the catholic Church's worldwide struggle to come to grips with the sweeping changes that the Second Vatican Council began 15 years ago.

Those changes envisioned lay men and women sharing with their priest the responsibility for their spiritual lives. And one result of the changes has been the increased participation by lay men and women in parish life.

At St. Mary's, however, some parishioners question how receptive the parish leadership is to any dissenting views. And so, gradually, a tug-of-war has developed.

The struggle has centered on the parish school where many parents claim, rapid turnover of lay teachers, unnecessarily harsh disciplinary methods and outdated religious teaching sabotage their chidren's education.

"If there's a Catholic left when they come of age it'll be a miracle,% said Alice Hopkins, a teaching aide at the school, who is the parent of a former St. Mary's student. Like the Duggans, she and her husband have withdrawn their son from St. Mary's.

St. Mary's pastor for 22 years, Msgr. R. Paul Repetti, dismisses the criticisms. "There are some parents you just can't satisfy," he said. "You have that situation all the time." As pastor, and thereby the person ultimately responsible for the parochial school, he said he was "completely satisfied" with the educational program.

The Washington archdiocese, to whom parents have appealed repeatedly, also has few qualms about the school. "We've never completely satisfied with any parish," said Auxiliary Bishop Thomas W. Lyons, archdiocesan vicar for education, "but I hink we've made sure about the quality of education at St. Mary's.

Many parents see it differently. They complain that religious teaching is out of step with the contemporary church. one 8-year-old came home from parochial school distressed because her mother sometimes serves as lector-or reader-at mass. She told her mother; "Sister says that if God had wanted women at the altar, he would have had Mary at the Last Supper."

Later the same child became upset when her mother kissed her during the "kiss of peace" at mass - a common practice in catholic churches today, "Sister says God doesn't like it when we kiss at mass," the child told her mother.

Asked about the incidents, Msgr. Repetti questioned the accuracy of the account, "I don't know whether Sister said that or not," he said, "but anyway, it doesn't make any difference because we have women lectors at mass."

Parents at St. Mary's, most of whom are products of the catholic schools an oppressive atmosphere hangs over the school - an attitude they find sharply at odds with what the American Catholic bishops have proclaimed as the hallmark of Catholic education today: "To Teach as Jesus Did."

Children, they say, are subjected to unreasonable demands. Seven-year-olds, for example, after sitting in class all morning, must remain at their desks to eat lunch, not even speaking above a whisper while they eat.

Parents complain that disciplinary practices are at best antiquated, at worst psychologically damaging and on occasion downright hazardous. Children are forced to "tattle" on each other. entire classes are routinely punished for the misbehavior of one student.

Entire classes have been repeatedly told that they are "no good" and "trouble-makers" according to the paretns.

On one occasion last spring the entire seventh grade was kept an hour after school because of the misdeeds of one class member, causing them to miss the school bus."

"This is an area of winding narrow country roads and no sidewalks and lots of traffic," said parent Tim Murphy. Murphy added that parents of the children, some of whom lived seven or eight miles from the school, learned of the detention only when they called the school after their youngsters failed to arrive on the school bus.

Murphy recalled that another time the same seventh-grade class was disciplined by confining it to the classroom while the local fire department conducted a specially arranged demonstration of life-saving techniques in the school yard.

Asked about these incidents, Msgr. Repetti said he sees no problems with the school's disciplinary procedures. "We do stress deicipline," he explained. "That's basic for the good of the children . . . There may be a few parents who pick at this or that. You have that any time."

But the amount of "picking at this and that" seems to be on the increase in St. Mary's, as more and more new parishioners join the old flock.

For more than 125 years, St. Mary's was a sleepy mission parish in a rural tobacco and truck farming area. But the urbanization which has swept Prince George's County in the last decade has changed that and St. Mary's is now a parish of some 1,200 families.

They come from both shanties and Potomac river-front mansions; tobacco farms and country club-centered developments of $150,000 homes.

Many of the newcomers ar Catholics from parishes that have caught the vision of Vatican II - of a community open to new ways of teaching the church's eternal truths, new modes of worship, new involvement of lay men and women in parish life.

Many of these families find the brand of Catholicism at St. Mary's, inboth the church and the school a disappointment.

Tensions are reflected in the fact that there havebeen seven principals in the 15 years the school has been in operation. Over the last three years, 15 teachers have resigned or been fired.

The situation at St. Mary's first boiled over more than 18 months ago when parents learned Msgr. Repetti was easing out a highly regarded lay principal and three tenured lay teachers, and replacing them with nuns.

Msgr. Repetti has said he sought to replace the educators with sisters because "any catholic school wishes to have a faculty of sisters. That's the way it's always been."

The pastor insisted that Rose Horman, the lay principal, had notified him she was resigning before he moved to find a nun relacement for her. Parents are equally adament in their insistence that Horman resigned in disgust when she learned the pastor was surreptitiously seeking to replace her.

In any event, the principal and the pastor did not part friends. in he second - and last - year at St. Mary's, Horman took on in addition to her own work, the direction of the parish regious education program for children not enrolled in the parish school.

For Horman, the combination of the heavy work load plus the tensions over the pastor's avowed intentions to replace lay personnel with nuns sent her blood pressure to dangerous levels and her doctors ordered a rest.

In February, 1976, she notified Msgr. Repetti of her need to take a month's sick leave. He refused, saying that the request "is not helpful to me or to the school and unacceptable for administrative reasons." Subsequently, horman left her post and instituted archdiocesan grievance procedures. As a result, the pastor was forced to pay her salary to the expiration of her contract.

The turmoil surrounding the departure of Horman and the three lay teachers galvanized the usually docile Catholic parents into action. Committees were formed, meetings were held and a veritable blizzard of letters went forth to William cardinal Baum of the Washington archdiocese. None of these, according to the parents, were even acknowledged.

A spokesman for the cardinal said it was his policy to refer parish complaints to the appropriate vicar - in this case Bishop Thomas Lyons, who did meet with a committee of parents several times.

Last spring, five more lay teachers - half the faculty - were fired or eased out of St. Mary's.

Mrgr. repetti said they were not forced out but "decided they would rather go somewhere else."

Meanwhile, said Murphy, "It's the children who are suffering because of the break in continuity of teachers, year after year."

Parents say their repeated efforts to improve or even discuss the situation are rebuffed. "The children have been told and we (parents) have been told, 'If you don't like the system, get out'," said Eileen Giglio, mother of two St. Mary's pupils and an active lay worker in the parish.

Bishop Lyons acknowledged that "there is a residue of some resentment" and dissatisfaction among some of the parishioners at St. Mary's, but he points out that "there is very strong support for the pastor too."

Even his severest critics at St. Mary's concur in the common assessment of Msgr. Repetti as a "pious, holy man," who, as Catholics, they respect.

But as parents, they cannot forget last spring, when their children came home from parochial school, reporting: "Sister said God has left St. Marys."