It's also a right of parents guaranteed by a long standing Supreme Court Ruling -
http://www.hslda.org/docs/nche/000000/00000075.asp
Here is an excerpt from this ruling that addresses Pierce.
http://www.bayareanewsgroup.com/multimedia/mn/news/homeschool_opinion_030708.pdf
“In obedience to the constitutional mandate to bring about a general diffusion of
knowledge and intelligence, the Legislature, over the years, enacted a series of laws.
A primary purpose of the educational system is to train school children in good
citizenship, patriotism and loyalty to the state and the nation as a means of protecting
the public welfare. [Citation.] The Supreme Court of the United States, in the case of
Pierce v. Society of Sisters, 268 U.S. 510 [45 S.Ct. 571, 69 L.Ed. 1070, 39 A.L.R. 468],
held that: ‘No question is raised concerning the power of the state reasonably to regulate
all schools, to inspect, supervise and examine them, their teachers and pupils; to require
that all children of proper age attend some school, that teachers shall be of good moral
character and patriotic disposition, that certain studies plainly essential to good
citizenship must be taught, and that nothing be taught which is manifestly inimical to
the public welfare.’ [¶] Included in the laws governing the educational program were
those regulating the attendance of children at school and the power of the state to
enforce compulsory education of children within the state at some school is beyond
question. (Meyer v. Nebraska, 262 U.S. 390 [43 S.Ct. 625, 628, 67 L.Ed. 1042,
29 A.L.R. 1446]; Ex parte Liddell, 93 Cal. 633, 640 [29 P. 251].” (In re Shinn (1961)
195 Cal.App.2d 683, 686-687.)