Keep in mind that over time schools merge, consolidate, close, change names, etc. Those administrative details will impact how you look for the school of interest.
The coverage is not 100% for all schools for all years during the time frame.
Some schools are not cataloged appropriately.
Not all yearbooks are indexed and those that are have issues.
Search “U.S., School Yearbooks, 1900-1990” for yourself or your relatives. You may discover hair you forgot you ever had.