J. Peter Byrne, Academic Freedom: A "Special Concern of the First Amendment" 99 Yale L. J. 251 (1989) (Depends a little on your area. I haven't been a big K-12 person, but am very interested in universities and academic freedom. There, I must recommend suggest Byrne; suggested by Paul Horwitz).
Paul Horwitz, Grutter's First Amendment, 46 B.C. L. Rev. 461 (2005) (although it's not canonical, I'll go ahead and plug my own; suggested by Paul Horwitz).
Education Law Books
Joseph P. Viteritti, Choosing Equality: School Choice, the Constitution, and Civil Society (2001)
(a favorite of mine; suggested by Rick Garnett).