The D.C. City Council voted yesterday to cut off all reduced fares after April 15 for suburban children who attend schools in Washington.

The action will cost suburban parents as much as $400 a year in additional bus fares for each affected child, while saving D.C. taxpayers as much as $750,000 in Metro subsidies, according to one rough estimate.

Anthony Rachal. head of mass fransit activities for the D.C. Department of Transportation, said most of the affected pupils attend private and church-sponsored schools. By what he called "a rough guess," as many as 12,500 pupils from the suburbs attend such schools in the city.

Yesterday's council action will required enforcement of a rule that only children who both live and attend school in the city may buy Metrobus tokens for 10 cents each. From tax revenues, the city pays the Metro transit authority an extra 35 cents for each token collected on the buses, a total of more than $4 million a year.

Before voting yesterday, some council members said they do not want to pay out money to subsideize suburbanities while those same suburbanites resist attempts in Congress by the District of Columbia to tax the salaries they earn in the city.

"When I think of . . . arguments against the commuter tax on the (Capitol) Hill, it's really an insult to us," exclaimed council member Hilda H. Mason (Statehood-At Large), who first proposed the ending of the suburbanites' bus subsidy.

Mason won council approval last month for an immediate cut-off of the subsidy, but Mayor Walter E. Washington sent aides to a council meeting yesterday to ask for more time to put the rule into effect. Council Chairman Sterling Tucker proposed the new April 15 deadline.

The new move to curtail the subsidy comes at a time when high school students in Washington are organizing support for subsidizing their rides on the Metrorail system as well as on the buses. They complain that the rerouting of the buses that accompanied recent expansion of the rail system is making their bus trips more expensive and circuitous.

The council's transporation committee has scheduled a hearing on a Metrorail school subsidy program for April 15. Quite likely it will also develop into a forum for complaints against the cut-off of subsidies to suburban pupils.

School officials reached yesterday said the cut-off probably will cause some problems, but not severe hard-ships.

The Rev. Clement C. Petrik, headmaster of Gonzaga High School, said about half of the Catholic institution's 560 pupils are suburbanites. "I don't know how they'll react," he said.

John J. Humphrey, dean of Emerson Preparatory School, a private institution, said about a quarter of the student body of 100 rides buses from the suburbs. "Anything that makes it difficult for students to attend school is not going to help D.C., especially when their coming in helps the economy of the city," Humphrey said.

Brother Paul Rahaim, vice principal of Mackin High School, siad students, including those from Maryland, are more concerned about getting a Metrorail subsidy.

When the school fare subsidy was introduced in 1962, it was intended to apply only to city residents, but the law was drafted in a way that permits its use by all pupils up to age 18 who attend classes in the city.

The subsidy was provided because Washington, unlike the suburbs, does not have a fleet of special school buses. As a result, thousands of city pupils ride buses on regular Metrobus routes to school.

City students who attend schools in the suburbs cannot qualify for the subsidy. And none of the suburban Washington counties and cities has a local school fare subsidy similar to Washington's. The suburban schools all provide local school bus service.

Meanwhile, the City Council postponed until April 4 consideration of a move by Nadine Winter (D-Ward 6) to remove initial funding for the city's proposed downtown convention center from the 1978 budget and put it into the 1979 budget.

The council also postponed until April 4 the enactment of a tax on the profits of real estate speculation.