[pyrophoric]

Compare bond strength of C-OH on the basis of  formation of free radical/carbocation  after homolytic /heterolytic bond cleavage.More stable is the radical or Carbocation more easy to break  bond or weak is  bond.I think this will help you in comparing Heats of Combustions of primary, Secondary and tertiary alcohols.

Is this really supposed to be an answer for a high school student? 

[Vidya]

If high school student wants to know the reason and student knows  about primary /secondary or tertiary alcohols ...then may be it is yes.