A House-Senate conference committee reached agreement over the weekend on a $475 million outlay that will save the nationwide federal impact aid program for schools, but will finance it at a reduced level.

The formula for apportioning the much-reduced funding has not been decided, so it was impossible yesterday to determine how much money will be available to school systems in the Washington area that have been traditional beneficiaries of the 31-year-old program.

Every president since Dwight Eisenhower has sought to curtail the program drastically, but failed until it become part of the Reagan adminstration's budget-cutting program.

In the current fiscal year, $725 million was appropriated nationally to pay for educating the children of federal workers, notably those of military personnel. Of that, $23 million went to schools in the Washington area, led by Fairfax Country with $9.4 million, Prince George's Countrarea, with $4.3 million, the District of Columbia with $3.6 million and Montgomery County with $2.2 million.

Prior to the conferees' agreement, the House had voted to grant $381 million and the Senate $500 million to keep the program going. Senate efforts were spurred by Sen. John Tower (R-Tex.), chairman of the Armed Services Committee, who feared a disruption of the education of children in miliary families. Three states, including Virginia, have passed laws permitting the charging of tuition under some conditions to such children.