"School newspapers and other publications bear the school's imprimatur." In other words:

b. Their content is officially approved by the school

Explanation:

If there are publications or a newspaper of some School then those publications cannot be published without approval or licence from School principal.

If their content is approved by the school, then it can be published and not without it because that content is written, printed and published in school. It is usually done by the students and they are writing about useful information, school activities and more that is happening in the school that they are in.