McGraw-Hill Cos. has signed an exchange agreement to acquire the Times Mirror Higher Education Group, the fourth largest college publisher in the U.S. with 1995 revenues of $228.2 million. In return, McGraw-Hill will trade its Shepard's/McGraw-Hill legal publishing unit to Times Mirror and will also receive additional undisclosed consideration. The Times Mirror education unit is comprised of Richard D. Irwin, Wm. C. Brown, Brown & Benchmark, Irwin Professional Publishing and Mosby College. Times Mirror, which already owns legal publisher Matthew Bender & Co., said it will put Shepard's into a new 50/50 partnership with Reed Elsevier as the first step in what is expected to be a broader strategic alliance between Bender and Reed-Elsevier's Lexis-Nexis subsidiary, which provides full-text online information services in the legal, news, business and government areas.