throw_away mentioned the joint venture, but its easiest to see the difference if you look at Macmillan's logo (nowhere to be found at mhschool.com), and maybe more importantly, Macmillan/McGraw-Hill's titles, which are all educational, and not quite what you'd expect from one of the big 5 advertised at today's Stevenote.

In fact, the joint venture ended awhile ago. Some context:

  [In] 1989, McGraw-Hill entered into a 50/50 joint venture
  with Macmillan, combining the elementary, secondary, and 
  vocational education businesses of both companies[...]

  [...] In 1993, the company bought out Macmillan's half of
  the Macmillan/McGraw-Hill School Publishing Company[...]

http://www.answers.com/topic/the-mcgraw-hill-companies-inc