I believe the answer is:
The income level of the student's household
The work schedule of the parent or parents
Hidden variable refers to the variable that technically not included as independent variable, but would create an actual perception that an individual variable somehow correlate with the dependent variable.
In the case above, children who do not have breakfast do not perform well at school because of the breakfast, but because family that cannot afford breakfast tend to consist of parents who do not have time to pay attention to the children.
