At Arlington's Yorktown High School, the school band has about 20 fewer members than it did a year ago, there is no longer an orchestra and the choir is all female.

Pamela Knudson, mother of six boys, two of whom attend Yorktown, thinks the situation is grim and admits it's something she gets emotional about. She says it's ''agonizing'' for her to watch her sons in high school, who are talented in art and music, be forced to choose between courses in one or the other. Other students must choose between a vast number of special art and music offerings.

Last Thursday, Knudson addressed the Arlington School Board, urging members to accept School Superintendent Charles Nunley's proposal to implement a mandatory seven-period day in the county's three high schools next September. Like many other parents, she feels that the addition of another class to the existing six-period day will give students the badly needed chance to take electives, especially in the fine arts.

''For more people it would be an opportunity to round out their lives,'' she said. ''That opportunity is something we plea for.''

The push for a seven-period day in Arlington originated from several sources. Its implementation was one of the suggestions made in the 1983 nationally heralded report on education, ''A Nation At Risk.'' The drive was also propelled by Arlington parents and educators, who grew more and more concerned that students had less time for electives with high school graduation requirements increasing from 18 to 20 credits.

Nunley's proposal to the board Thursday suggests seven 50-minute class periods and calls for the addition of 44 teachers at an estimated cost of $1.1 million.

Although board members said they are generally in favor of the plan, they criticized Nunley's proposal for its lack of concrete detail and appeared baffled as to how the program could be actually implemented.

''I really find myself on board with the seven-period day,'' said School Board member Simone Pace. ''The only thing I'm really hung up on is length of the day.''

According to Arlington schools public information specialist Dennis Smith, the academic school day for high school students in Arlington currently starts at 7:30 a.m. and ends at 2 p.m. Periods vary in length from 52 to 55 minutes, intervals between classes last five minutes and there is a 50-minute lunch period.

Nunley's proposal calls for the school day to include another period without substantially changing the length of the school day. At the board meeting, Nunley said the day could be restructured to reduce time between classes, shorten the lunch period and add about 20 minutes to the beginning or end of the day.

But board members expressed concern about beginning he day earlier, agreeing that it starts awfully early anyway. At the same time, they said that a later dismissal would cause transportation problems and would involve additional costs. (Currently, intermediate school and elementary school students are dismissed later than high school students, which allows a limited number of buses to make several trips. If more students left at the same time, the school system would have to obtain more buses, a cost factor not figured into Nunley's report.)

''It seems to me we have a lot of questions we need answers to,'' said School Board Chairman Gail Nuckols. ''We need a lot more information.''

Majorie McCreery, executive director of the Arlington Education Association, agreed that the proposal was too vague. ''The superintendent's recommendation is not well articulated enough for staff people to see how it's going to affect them,'' she said. She explained that the association was not against giving opportunities to students but that there ''would be objections to expecting these opportunities to come as a charitable gift from your staff.''

Nuckols expressed frustration, voiced by other board members, that the board had not been presented with a more concrete package, especially since board members are in the process of wrestling with the budget and students need to set up their fall schedules in the next couple of weeks.

''I want to know how much it's going to cost,'' she said. ''I want to know what the bottom line is.''

The board asked the school administrative staff to come up with a ''model'' for implementing the plan by the next board meeting Feb. 7. Members asked that the model include starting and ending times of the school day, length of class periods and a detailed outline of all costs involved.

''I think we have to know what we're signing up for,'' Nuckols said.