Currently, a prospective student's "HS Grad. Date" is specific to the lead info set. Thus, when we have individuals with multiple lead info sets (i.e., they were prospects for one or more degrees over multiple periods of time), the HS Grad. Date has to be populated for each lead info set to avoid discrepancies / mismatches / errors.
Since each individual only graduates from high school once (at least, so far as I know), it would seem better to have the HS Grad. Date be a permanent, one-time-entry Admissions field, similar to Birthdate in Person Info. This field could be located in the top-right corner of the individual's Admissions tab above any Admissions Custom fields, similar to how built-in Person Info fields appear above Custom Info fields.
I recognize that an individual could have a lead info set from before they graduated from high school and one or more lead info sets added after they graduated, in which case the school might need to do a little cross-referencing to determine the individual's status at each stage, but it should be easy enough to include the real-time field status in exports to be retained in static form, thereby retaining the historical data.