THE ACADEMIC grade known as the "gentleman's C" takes on a new and unfortunate meaning when it is applied to how well the various states perform in collecting child support payments. It was the best grade that any state was able to earn in the nationwide rankings just released by the House subcommittee on human resources. Locally, the situation was even worse. The District of Columbia finished 50th out of 51, surpassing only Oklahoma in reducing welfare costs, in paternity establishment, collection rates and efficiency. Virginia finished 31st, with a "D," and Maryland 49th, perhaps because it wasn't even able to collect its information in time for the study.

The subcommittee's report is the most recent and obvious signal of how far the nation has to go on behalf of its custodial parents, particularly when one considers the fact that both the District's and Virginia's scores represent improvements over past performances.

In Virginia, for example, officials managed to collect $121.4 million in child support in fiscal year 1990, up from a paltry $19.3 million in fiscal year 1986. That was due to a variety of efforts, including the use of collection agencies to track down delinquent parents, the publishing of a "ten most wanted" list of the fathers and mothers who owe the most and the fact that hospitals have begun asking fathers to sign forms voluntarily acknowledging paternity.

The situation is far worse in the District, where relatively new child support enforcement guidelines may help. There is still a backlog of 19,000 child support cases in Washington, and the city has already been fined more than $5 million as a result. One step in the right direction would be legislation that gives a voluntary acknowledgment of paternity the same legal weight as a court judgment.

The House subcommittee on human resources could also help by producing a report that goes beyond sheer numbers and rankings. It would be useful, for example, for other states to know more about how Texas and Delaware managed to collect child support at rates that doubled the national average. How do officials in Iowa and Connecticut manage to establish paternity in 80 percent of their welfare cases? Why are Maine and Indiana the best states in terms of the cost effectiveness of their efforts? If those successes could be duplicated, perhaps a few states might even manage a "B" in next year's rankings.