Perhaps it would be easier to know the situation if we had a War Room map showing the status of each school district, but for the moment all we have to judge by are anecdotes from outraged parents. Their districts have informed them either of barriers being imposed to children seeing the speech or that there will be no viewing at all, and their response is (quite rightly) to be outraged. However, from most of the comments I've seen, most of these parents are either unsure of how to proceed, or incorrectly believe that their only options involve politely expressing their views in letters, emails, and phone calls. But I would say that, however trivial the matter may seem on its face, we should not be content to be reasonable on this.
The motives of those seeking to sabotage this utterly innocuous presidential speech are clear, as is the fact that they will use it as a basis to extend their reach even further into our society. They will badger and bully weak bureaucrats into making every little facet of civic life subject to the prior approval of their extremist agenda, until officials are afraid to allow anything these fringe lunatics might object to without first seeking their permission. Those weak bureaucrats will not be "unbullied" by hearing any number of reasonable arguments and "I respect your decision, but I disagree" comments. They are not doing this because they've been persuaded to do it, they're doing it because they're afraid of angry wingnuts. That or they are wingnuts themselves.
Either way, the solution is clear: Make powerful, intelligent, and solemn arguments that treat everyone hearing them with respect, but at the end, make one thing very clear: You will not tolerate any outcome other than the president's speech being shown, because their decision not to show it has no basis and is unacceptable. There is no legitimate difference of opinion on the matter: You are right, period, and it is the self-evident duty of school officials to put the children first rather than allowing extremist bigots to dictate educational policy. If they lack both the civic responsibility and professional integrity to do what is obviously right and clearly in the best interests of their students and their country, make clear that you will have zero confidence in them, and will be evaluating your options as a citizen to see them removed from their positions.
Say this all as calmly and without anger, but with cold intensity, so they know that you are as serious as can be. That is, you are not just issuing threats in anger - you will make it a personal mission to get them thrown out if they cannot protect your children's education from the bullying of a handful of nutcases. Mean every single word of it. Let them know that you think their actions are setting the stage for future ones that endanger your children's future and that of the country, and that they will quite simply have to exercise responsibility and do the right thing. Make clear that there will be no compromises on this, because there is no legitimate basis for the objection in the first place. If the wingnut parents don't want their children hearing from the President of the United States, they can tell the school to have them leave the room. Period.
This is your school district. Yours. You are not a supplicant requesting a favor. You are not a peer looking for friends. You are not a lawyer making a case, although you should certainly argue powefully and persuasively. You are demanding basic responsibility and intelligence from officials and authorities to whom you entrust the education of your children, and they will either deliver or you will seek their removal.
Addendum: I would also suggest you demand a public apology from these officials.
Addendum #2: Whatever the outcome, we should not forget about this. The officials who required permission slips or decided not to allow the speech to be shown should be hounded for either an apology or a resignation.