The suggested minimum requirement for mastery in writing by the end of second grade consists of thirty-five words.  These are known as "No Excuse Words."   The words listed below are in frequency-of-use order.   Third graders should write the following words correctly.

the you are this had
of that as from not
an it with I but
a he his have what
to for they or all
in was at by were
is on be one then


R. Sitton's Sourcebook for Teaching Spelling and Word Skills 2006