THE REV. Allan Boesak is in his second week of solitary confinement in a South African jail.

On Friday he was permitted a visit from his wife, who said he was in good health and spirits. But, in keeping with the Draconian laws that define the apartheid system, he is being held without charge or access to a lawyer. The government evidently permitted him a family visit in order to cut off any speculation that harm might be coming to him while he was in custody. From the visit, his wife also returned with his plea to mixed-race students in the western Cape region, who have been in the streets, "to restrain themselves in order not to be exposed to further violence."

Precisely here lies the terrible irony of locking up opponents of apartheid such as Allan Boesak. He was arrested for undertaking to lead a peaceful march calling for the release from prison of Nelson Mandela. He was attempting to direct the passions of the frustrated and angry black majority into political channels. Mr. Boesak is founder of a national community-based movement, the United Democratic Front, with the potential for providing a framework to speak for blacks and to allow whites a reliable independent conduit to them.

It is characteristic of Rev. Boesak that, in his statement from his jail cell, he called not for defiance but for "restraint." A white leadership with a clear sense of its obligations to the white community, not to speak of its obligations to other South Africans, would be responding gratefully to this sort of opening for calm and dialogue. The growing dimensions of the country's crisis, as revealed just in the brief period since Mr. Boesak's incarceration by new blows to the economy, should have underlined the need.

Instead, P. W. Botha's government retreats ever deeper into the laager, the tight armed camp. Its latest cruelty is to close indefinitely half of the public schools for students of mixed race in the western Cape; schools have been scenes of repeated clashes between mixed-race youths and police since Mr. Boesak's arrest. "Many schools were no longer performing an educational function but have become bases for planning unrest activities," the minister for mixed-race education said. Apparently he believes the students whose classes have been closed will sit quietly at home writing in their copybooks, if they have any.

And Allan Boesak, a compassionate and peaceable man, sits in jail, denied both his liberty and the opportunity to contribute to ending the heartbreak in his land.