Found OBHS Register - I think it only helps in a negative way
Dates are entrance year - around age 13 
Let me know if you want any details from the body Most entries provide a "current" address and occupation also date and place of death where known
These are all the Hendersons up to 1938
Otago Boys' High School  
Old Boy's Register  1863 - 1938 inclusive
Published Dunedin 1953 
Index P 342 column 1       (scanned with quick check)
  Hende. Keith B.      1942
  Henderson, Allan           1923
  Henderson, Allen J. M.   1931
  Henderson, Angus     1903
  Henderson, Art. J.           1915
  Henderson, Bruce K.   1931
  Henderson, Chas. S.   1928
  Henderson, C. B.           1929
  Henderson, C. S.           1933
  Henderson, David R.   1926
  Henderson, Eric C.           1924
  Henderson, F. McG.   1917
  Henderson, Fred. C.   1924
  Henderson: Fred. I.   1944
  Henderson, Geo.           1934
  Henderson, Geo. A.   1917
  Henderson, Geo. A.   1884
  Henderson, Graeme   1943
  Henderson, Herbt.           1891
  Henderson, Hunter,   1898
  Henderson, Hugh R.   1916
  Henderson, Hugh W.   1934
  Henderson, Ian A.          1938
  Henderson, Jas.       1863
  Henderson, Jas.      1902
  Henderson, Jas. A.           1916
  Henderson, Jas. G.           1927
  Henderson, Jno.      1894
  Henderson, Jno. H.     1947
  Henderson, Jno. R.    1924
  Henderson, Jno. S.    1906
  Henderson, Jos. W.   1924
  Henderson, Kevin H.   1946
  Henderson, M. C.      1887
  Henderson, M. R.       1932
  Henderson, N. D.       1926
  Henderson, Robt. B.   1912
  Henderson, Robt. H.   1917
  Henderson, Robt. R.   1926
  Henderson, Ross S.   1941
  Henderson, Wm.       1863
  Henderson, Wm.           1879
  Henderson, Wm. E.   1926
  Henderson, Wm. J.    1891
  Henderson, Wm. J.           1915
  Hendra, Henry N.           1926