A Washington real estate operator who once taught mathematics in the Montgomery County public schools was orderec yesterday to pay fines totaling $12,000 for failing to file D.C income tax returns.

The fines were levied by Superior Court Judge John Garrett Penn against David E. Bell, of 3710 Warren St. NW, after Bell pleaded guilty to willful failure to file personal returns for the years 1970, 1971 and 1972. Penn also gave Bell a three-year suspended jail term and place him on probation for five years.

Assistant Corporation Counsel Kenneth Pels said it was the largest fine imposed for D.C. tax violations in recent years. He said the case was part of a crackdown on District of Columbia tax violators that had been prompted by criticism in Congress and elsewhere of enforcement of these laws.

Pels said about 70 cases against individuals have been opened since the drive against suspected tax evaders began about nine months ago.

"It is important that people understand that these laws will be enforced," Pels said.

The maximum penalty that could have been imposed on Bell, who resigned from his teaching position in the Montgomery County public schools in 1974, was a $5,000 fine and one year in prison for each of the three years in which he willfully refused to file income tax returns with the city, Pels said.

He said Judge Penn ordered that the fines - $4,000 for each of the three years in question - be paid by June 15. The Judge also ordered that Bell pay any taxes due for the three years involved.

Pels said the exact amount of taxes due was not known because Bell has not filed complete returns, but that city officials had told the court that they estimated the amount to be $32,000.

Penn ordered Bell to fine complete tax returns by May 3.

Pels said Bell told the court he had properties in the city valued at $992,000. The prosecutor said this was in accord with city property records, which showed that Bell owned 15 properties in the District with a total value of $1 million. Of the 15 properties, 14 are for rent, he said.

As conditions of probation, Penn directed that Bell remain in the Washington area and that he pay all past and future taxes that he may owe to the District. If he fails to pay the $12,000 fine by June 15, he faces a year in prison, according to D.C. law.