(Not a Teacher)
I've never heard of 'yard duty staff' at an elementary school. Makes it sound like a prison. I assume they're faculty assigned to watch the children in the school yard.
Colloquially, "yard" can refer to a prison's exercise yard, hence my previous comment.
'Shaken', when referring to a person, means startled and a tad frightened. The boys probably realize they're in trouble now.
Student or Learner