WILLIAM J. BENNETT, secretary of education, asked 100 business leaders and college presidents recently to help him figure out how to keep kids in school. "Write me," said Mr. Bennett to his New York audience, "if you have found a way to motivate students to stay in school and go on to college, or if you know someone who has." As an example, he praised Eugene Lang, the millionaire who inspired a sixth-grade graduating class in Harlem by promising to pay full tuition for their college educations. Five years later, every child in that class who is still in the New York area is still in school -- a dramatic contrast to the area's 40 percent dropout rate.

Mr. Bennett told the audience he found it "striking" that Mr. Lang' promise motivated kids so much more strongly than federal financial aid does, even though "the actual money originally offered to these students is approximately the same as the amount students from low-income families can receive from the federal government's Pell Grant and other aid programs." The difference, he concluded, was Mr. Lang's "personal encouragement and support."

Mr. Bennett is right that encouragement is important, but there are problems with his broader diagnosis. First, Mr. Lang's hugely successful arrangement with the sixth-graders is hardly comparable to federal aid programs. Contrary to Mr. Bennett's assertion, Mr. Lang did not just offer each student "$2,000 toward their education" but the whole thing. (The maximum Pell or direct grant is $2,100 -- toward annual expenses that averaged $4,881 at public colleges and $9,027 at private ones last year.) To get full tuition from the federal government, by contrast, a student supplies family financial information. Colleges then clculate, based on standard formulas, the maximum amount they expect the family to contribute; the maximum they expect a student to contribute through student loans, summer jobs and government-aided "work-study" jobs; and Pell and other grants for the rest.

More important, the guarantees of even that much aid are precarious and getting more so. Mr. Bennett will not have forgotten that he himself led the charge against them last March, calling for a $4,000 "mega-cap" on federal aid per student per year, and saying that full aid based on need was "unreasonable." Efforts to cut will be redoubled in 1986, with the whole Higher Education Act up for reauthorization and the Gramm-Rudman balanced- budget legislation mandating cuts across the board.

Mr. Bennett's stance on all this will be crucial -- which brings us back to the matter of encouragement. Low-income high-schoolers now certainly realize that, no matter how much somebody encourages and believes in them, their chances of paying for college are getting dimmer. That the general climate of opinion on aid does affect poor and minority kids' aspirations became clear after the 1982 cuts, when the numbers of college applications from such kids dropped sharply. Since last year, Mr. Bennett has moderated the sharp tone in which he called for "stereo divestiture." Now would be a good time for him to focus serious attention on one very rational cause of student discouragement -- the growing chasm between rising college costs and shrinking aid sources. Money without personal encouragement, as he told the audience, may not be enough to keep kids in school. But for those not visited by a Eugene Lang, all the encouragement in the world is useless without money.