Perhaps there could have been some backup questions asking about the appearances and preparation of substances which could have been helpful in weeding out many of the false returns. Additionally, it would be better if students could fill out their questionnaires in isolation rather than in a classroom with friends looking over one another's shoulders. For an example of a Gallup poll questionnaire used in the lead up to the 1984 US Presidential elections see Freedman et al. The majority of the questions were designed to help the Gallup people predict who would and who would not vote. There are usually far more people with these ages in surveys than in the populations that they were drawn from.
Old people tend to overstate their age - advanced age being a source of status.
Hunters tend to exaggerate their daily catches when they are low, or they can be genuinely forgetful. Poor memory combined with overstatement can lead to a catch being reported as a number ending in zero or five, and somewhat above the true figure. However, the devastating discovery of cancer tends to fix the attention of the victims on their past and sharpen their memories in a way that doesn't occur for the controls. This can cause severe biases on items that require recall such as past dietary habits. Very often people want to take data from one population and transfer the results to another.