In the large, noisy cafeteria of Northwestern Senior High in Adelphi, where about 800 students lined up Monday for lunch in three shifts signaled by ringing bells, the hot meal of the day was "barbecue sandwiches." Two ounces of pork and beef on a hamburger roll, half a cup of potato and a quarter-cup of lettuce and tomato, a slice of chocolate cake and a half pint of milk.

It was a typical meal, the students said: too small and, at 95 cents this semester, too expensive.

The price of school lunches went up 20 cents in September, and already there has been a marked decrease in the number of students eating school meals, Prince George's County school officials say.

The number of children lining up for school lunches is expected to fall further when new free and reduced-price lunch qualifications come into effect in October. A family of four must earn less than $10,990 per year to qualify. Last year, the limit was $10,270. But Prince George's County expects between 10 and 15 percent of those on free and reduced-price programs will be ineligible because of increases in family earnings. Last year, 32 percent of the county's children qualified for these lunches.

C. Anthony DiMuzio, director of the county schools' food service, said he expects fewer children to line up for lunch if they get less food for their money. The Department of Agriculture wants to let schools serve portions even smaller than those at Monday's lunch.

DiMuzio said reduced participation will make it very difficult for cafeterias to operate within their budgets. The Maryland Department of Education, worried by the number of students dropping out of school lunch programs, is planning a billboard campaign next month to encourage more students to eat meals at school.

DiMuzio said he is worried that many students already are not getting enough to eat. "I'm constantly being told by teachers that this is the only full meal and hot meal that they (the students) get in their diets," he said.

But "simply to save money," he said, he will order less food served if he is allowed to do so. Children will probably get six ounces of milk instead of eight, he said, "and we won't have to put as much fruit and vegetables on." He said this will probably save between six and eight cents on each meal served.

"Now that's very good on the economic side," he said. "But are we taking care of the physical needs?" DiMuzio, who believes the answer is no, said most students eat all the food they are given, and that he would not reduce portions unless he had to. But he said he is trying to operate an efficient, self-supporting program, and added, "I'm caught in the middle. Obviously, I'm going to have to" reduce portions.

At Northwestern, complaints already have been heard. "They give us so little and they charge us so much," said 10th grader Roy Croll as he finished his barbecue sandwich. "I'm still hungry."

Richard Germany, a senior, said he is always hungry before he gets home. "If they give us less, I just won't buy," he added. "I'll cut lunch and go to McDonald's." With the increased prices, Germany said, more students are risking suspension by leaving school to eat at the fast-food restaurant.

Wally Lee, a 12th grader who said he was on the free-lunch program, said he ate a school lunch but also brought food from home. "It doesn't fill me up," he said. "The only reason I eat it is because it's free."

Vice Principal Toni Menshan, who supervised the lunch period on Monday, said most of the children eat only "because it's that time of the day," and that many of them do not eat enough. "They could eat enough if it was given to them," she said.

Each day she buys at least two lunches for students, sometimes for teen-agers who have forgotten their lunch money, she said. But sometimes they are extra lunches for students who have eaten but are still hungry. "Some of the kids don't receive anything to eat at home," she said, "and what they receive here is fairly minimal."

Vera Macon, an 11th grader, finished everything on her plate as quickly as she could. "This is the only day it's good, and that's because I'm hungry," she said. "It's not usually good." She said she usually buys ice cream, cookies and soda from the snack bar, and eats dinner as soon as she gets home. "They don't give you nearly enough to eat," she said.

Statistics aren't available for this semester, but school system spokesman Brian J. Porter said there is no doubt fewer students are eating the lunches because of the price increase. "We found, intitially, there is a drop-off of people eating lunch," he said. "Raising the price by a nickel, which we did before, and then again by 20 cents, means that more kids are going without."

Porter said he is also concerned about the possibility of students receiving less food. The Agriculture Department's proposal reduces the minimum portions schools must serve, allows condiments, such as ketchup, to take the place of a vegetable or fruit, and meat substitutes to take the place of meat.

"That struck me as absurd," Porter said. "Kids need bulk. They need food, they need fiber. Having time to eat a good solid meal is important. For some of our kids this is the only meal they get -- the only meal that is nutritional."

Beatrice Largay, a nutrition expert with the county schools, agreed that students would not get enough food. "For high school students, for adolescents -- they demand a lot of food." She said a survey of 2,000 county students she conducted last year found that between 40 and 45 percent of them did not eat breakfast and another 30 percent ate it irregularly.

"If lunch prices go up, then they don't eat lunch," she said. "Then we're in trouble." The higher prices will mean that "probably fewer children will eat," she said. "Those who don't eat the school lunch probably won't bring their own lunch, either."

Robert Ambrose, principal at Mount Rainier Elementary, said the cafeteria at his school has been serving about 120 lunches daily this semester. Last year, he said, the average number was 200. The number of breakfasts served has fallen from about 90 to 36, he said.

"A lot of parents have told me they can no longer afford to buy three kids lunch," he said. "The children are being provided with food at home or they are not getting it." But he said he didn't think children at his school went hungry. Most of them walk to school, he said, and can walk home for lunch.

Thelma Butler, principal of Capital Heights Elementary, said more children are bringing lunches from home this year. "I know my son (who goes to Bowie Senior High) used to buy lunch. Now he's bringing a bag lunch."

While she said no children will go hungry, "it's going to affect the kids." She said the cafeteria manager at the school has told her that many children depend on the school for their lunches. "It's going to be very hard," she said.

Gus Crenson, public information officer for the Maryland Department of Education, said the state has lost about $6.5 million in federal aid for school lunches this year. "There's apprehension that the higher prices will drive kids out of the school lunch program," he said.

In response, he said, the state Department of Education will begin a billboard campaign next month with the message that "a school lunch is still nutritious." Crenson said space on about 40 billboards across the state has been donated for the campaign.

Jeanette Gordy, director of the Prince George's Head Start Program for pre-schoolers from poor families, said she is most worried about children who will no longer be eligible for reduced-price and free lunches. Lunches are provided children in the program in the same way they are at schools. Gordy said already there are too many hungry children in the county.

On one occasion last year, she said, a boy in the program sat down to lunch and "ate, ate, ate. Pretty soon he threw up. It was a Monday I shall never forget: the boy hadn't eaten all weekend."

On another occasion, Gordy said, the mother of a child in the program told her the only meals her son received were those given him at Head Start.

"The kid got one meal a day," Gordy said. "You cannot slough that off as an isolated incident." Gordy said the mother did not feel guilty: at least she knew her son received one good meal a day. "That kid is in kindergarten this year. What category is he going to fall into? Even if he gets a reduced (price) lunch, where is the mother going to get the money from?"