Boston's school bus drivers returned to work relatively smoothly yesterday after a three-week strike, although a spokesman for them said additional disagreements have developed with ARA Services, Inc., which provides the buses.

Kara A. Spelt, the spokesman for Local 8751, said that ARA yesterday improperly fired one bus driver and gave stand-by runs--normally assigned to union drivers--to drivers who had substituted during the strike. Nevertheless, the drivers will not strike again, she added.

John W. Black, a regional president of ARA, said yesterday the first day after the strike "went about normal" with only a "little bit" of confusion and lateness.

The bus drivers decided Sunday night to return to work after agreeing with ARA to submit disputes to "mini-arbitration." Settlements will be handled by local arbitors, who are cheaper and faster than a national arbitration association, Spelt said.

ARA also agreed to return striking drivers to their previous routes, to rehire 19 union leaders after a 30-day suspension, and to discuss some 19 additional union demands. Any demands not resolved within one week will be sent to expedited arbitration.

"They've been using our contract like it's toilet paper," she said, adding that discussions with ARA could not achieve anything.

Bus drivers hired during the strike to replace striking drivers will remain on the company payroll, Black said.

A primary area of dispute involves the amount of time given to the drivers for safety checks of the buses. Spelt said that the roughly seven minutes allotted drivers twice daily to punch in, walk to the bus, check indicators, reflectors and lights, write a bus report, and start up is not enough. Last year the drivers had 15 minutes for each check, she said.

The state mandates 15 minutes checking per day, Black said.