Here's some case law, but it still doesn't define profanity.

"There are certain well-defined and narrowly limited classes of speech, the prevention and punishment of which have never been thought to raise any Constitutional problem. These include the lewd and obscene, the profane, the libelous, and the insulting or 'fighting' words . . . . It has been well observed that such utterances are no essential part of any exposition of ideas, and are of such slight social value as a step to truth that any benefit that may be derived from them is clearly outweighed by the social interest in order and morality." ( Chaplinsky v. New Hampshire (1942) 315 U.S. 568, 571-572 [86 L. Ed. 1031, 1035, 62 S. Ct. 766], fns. omitted; accord, FCC v. Pacifica Foundation (1978) 438 U.S. 726, 746 [57 L. Ed. 2d 1073, 1091-1092, 98 S. Ct. 3026]; Lee v. Superior Court (1992) 9 Cal. App. 4th 510, 517 [11 Cal. Rptr. 2d 763].)

This reasoning is particularly apropos when the expression is aimed at minors rather than adults. (See FCC v. Pacifica Foundation, supra, 438 U.S. at p. 749 [57 L. Ed. 2d at pp. 1093-1094]; Ginsberg v. New York (1968) 390 U.S. 629, 638-640 [20 L. Ed. 2d 195, 203-204, 88 S. Ct. 1274].)

Censorship of "four-letter words" does not unduly hinder the students' ability to express their ideas or opinions on any subject. It enjoins only the indecent manner in which an idea is expressed. [*1326]

"A requirement that indecent language be avoided will have its primary effect on the form, rather than the content, of serious communication. There are few, if any, thoughts that cannot be expressed by the use of less offensive language." ( FCC v. Pacifica Foundation, supra, 438 U.S. at p. 743, fn. 18 [57 L. Ed. 2d at p. 1089].)