I spent a little while in the state library today.  Would you believe 1985?  That was when the law first compelled all NH 14 and 15 year olds to attend school.   There were various amendments to the age 14 requirement between 1903 and 1985, but as late as 1985 students who had no high school in their district were not required to attend school beyond 14, unless they hadn't yet completed elementary school.  And parental freedom to remove a child, even before age 14, was quite extensive, though as you might imagine it was being whittled away over the years. Will try to put together a good chronology before next Tuesday.  
    I don't recall at the moment when 36 weeks for all elementary schools went into effect -- maybe 1921 -- there were massive education law revisions around then.