USUALLY YOU interview for a job with your prospective employer. But William Bennett and John Silber, prospective candidates to become U.S. secretary of education, found themselves last week climbing the steps of the Capitol Hill townhouse headquarters of the Free Congress Research and Education Foundation to be interviewed not by the administration but by representatives of a dozen or so conservative groups.

It doesn't seem likely that it was their own idea to go there. "We knew," said a White House staffer, "there were certain conservatives who wanted to meet with the candidates. We told the candidates there would be no problem if they did." Would there have been a problem if they didn't?

What is going on here looks very much like a policy of subcontracting government -- or a sizable hunk of it -- to the private sector. To which you may say, it's done all the time: think of the AFL- CIO and the department of labor in Democratic and some Republican administrations. The reply to that is that this sort of thing is seldom done so explicitly. And even if it's hard to say exactly where the line should be drawn, this lies on the wrong side of it. To which you may go on to say, many of the goals of these conservative groups are unobjectionable, in line with those of the big education commissions of 1983, and far to be preferred to the goals of some of the pressure groups that had an in with the Democrats in this department (and even got it created in the first place). Well, there's something to all of that. But it doesn't justify this sort of interrogation by the guardians of ideological purity.

The best thing that can be said more or less in defense of this interviewing process is that the existence of the department of education invites abuse. It was created by the Carter administration and a Democratic Congress solely to give the political entrepreneurs who ran the National Education Association a trophy to put on their mantle. It never had any business as a separate department, since the federal government's role in education is very limited. Salutary innovations have percolated upward from the nation's 15,000 school districts, never downward from the department. Yet its existence invited first the NEA lobbyists in the Carter years and now certain conservative lobbyists in the Reagan years to wage strenuous battles, all out of proportion to what is at stake, for control of the department's leading positions. The vetting of prospective secretaries over at the Free Congress headquarters is a characteristic part of this dismal process.