Teacher Spectre Famous Photo

A now famous photo of possibly a teacher is recorded on an English playground in 1977. Is this a spectre of behind the children?

Interestingly, the unknown female is see-through and moving, but nothing else in the photograph is blurry like this.

Is she a ghost in the picture, appearing behind the children in secret? Could she have been a former, deceased playground monitor still at work?

Photographed by a Mrs. Gregory, she remained convinced that there was no one visible behind the children during their Silver Jubilee celebration.

Another photograph taken prior to this one did not show anyone near the setting. So, where did the phantom teacher come from?