Arlington's cable television system has requested county approval of a 6 percent increase in basic rates and permission to replace programming on two of the six channels reserved for the schools and county government.

Arlington Cable Partners, which acquired the county cable franchise for $33 million last December, asked that the County Board approve the changes by Dec. 1 and make them effective Jan. 1. The board can reject a rate increase, but has not done so in the six-year history of the system, which serves 31,500 of the county's 80,000 homes.

John D. Evans, president of the system, said in a letter released yesterday that the company is proposing a 75-cent increase to the current monthly rate of $12.50 for the basic 35-channel system. The firm received a 95-cent rate increase last December.

The company also is planning to increase the monthly subscription charges for the Home Box Office channel by 25 cents from the current $11.25 rate, and by $1 for the Disney channel, now $9.95 a month, Evans said.

He noted that recently enacted federal cable television legislation provides for automatic approval of up to a 5 percent annual increase on basic rates for the next two years, and for the elimination of all rate regulation afterward.

The cable company is also asking to transfer one of the four school channels to a new channel designed for school use only. It would not be available to most subscribers as are all school-controlled channels.

The company also wants full use of one of the two county government channels, but would allow the county to reclaim one channel with a 180-day notice. The county consolidated all of its programs on one channel a few months ago, leaving the second blank.

Evans said the company wants to put Christian Broadcasting Network programming and an arts and entertainment network on the unused county and school channels.

"It doesn't do the public any good to withhold channels when we're not going to be using them," County Board Chairman Ellen M. Bozman said yesterday. She predicted the board will approve the company's requests.