I think Macmillan/McGraw-Hill was just a joint venture they have in elementary school textbooks. Macmillan is private, while McGraw is public, so I don't think either is owned by the other.
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[...]
