NATIONAL COMMISSIONS more often ratify changes in the country's thinking than lead it. The value of the report a few days ago by the Commission on Excellence in Education is precisely that it marks a point already reached by a great many people--a large majority, we should guess, of those who take a serious interest in the schools. There's a wide consensus that this country has become depressingly tolerant of mediocrity in its school systems, and the commission offers several suggestions. It could have gone a good deal further.

Lengthening the school day and the school year won't achieve much by themselves. Raising teachers' salaries won't change things, unless the raises are used vigorously as levers to introduce higher standards. And it is futile to give English teachers five classes daily of 25 students each, and then complain that they do not assign enough written homework--to be corrected and graded, presumably, by the teachers over the weekends.

The commission might usefully have gone a step further and considered a couple of mildly radical propositions. First, perhaps American high schools have gone too far to dissuade youngsters from dropping out. With the best of intentions, schools have left a lot of kids with the impression that it's somebody else's responsibility to see that they graduate and somebody else's fault if they don't.

Next, teachers need better control of their classrooms. Even high salaries won't induce enough good teachers to work in schools that fail to help them enforce the basic rules of behavior and respect for learning. To do the better job that the national commission urges, teachers are going to need more authority than most school systems--fearful of collisions with parents, wary of litigation--have been giving them.

Just under the surface of every report like this one, there's the question whether higher standards mean less equality. But this country's experience is strong evidence to the contrary. Good schools have always been the way up--and good schools don't let kids fall behind simply because they come from the homes of poor and uneducated parents. Conversely, slack and complaisant schools don't contribute anything to equality of opportunity when they treat indolence and diligence as though they were the same. It's hard to imagine a more misleading preparation for adult life in a competitive world.

President Reagan tried to deflect the thrust of this report with a homily on tuition tax credits and other irrelevancies. But the commission is calling for more forceful prosecution of public responsibilities, with more public money. It's correct to say that money alone can't do much. But few of the commission's purposes can be accomplished without more money.