I could make an argument for both "Readers' Digest" and "Reader's Digest" and to be honest, without Googling it, I couldn't tell you which one the publication uses. The plural suggests the many readers of the magazine and the singular could be said to make each reader feel much more individual by referring to them as "you, the reader".
However, "boys' school" makes far more sense. You don't have a school with just one pupil, who happens to be male. The school is attended by "boys" so it is a school "for boys" so it is a "boys' school".
Student or Learner