The two have nothing to do with one another. Sure the accused is entitled to a defense and no one should mess with that right, but that does not mean a defense attorney must be entitled to speak at a school. The parents and students have every reason to get a speaker they want to hear for their special day, and there's no rule saying that they must invite a certain person or that they cannot rule someone out based on his/her career and publicity choices.
I disagree; they rescinded the invitation based on his career as a defense attorney. Had they never invited him to speak in the first place, then there wouldn't be an issue.
I hope that the persons who complained and got the invitation rescinded, never have need of an attorney's services. I almost hope that local attoneys do take note and decline to represent them in the future. Let them reap what their hostility to the legal process and its actors has sown.