This is a complex issue from both the perspective of halachah and from the perspective of school policy. Let's begin from the policy perspective. I beleive that an Orthodox program can serve students who are not from the Orthodox community. However, this does raise the issue of how can the Orthodox school demonstrate respect for and sensitivity to the practices of families and students who come with norms of practice that differ from the Orthodox norm. There is no easy answer. Also, different responses may be appropriate for different schools. As a general rule, schools must consider not only the substance of their response but also how it is likely to be perceived. I will use this case as an example. My understanding is that SAR is implementing an accommodation for a set of students coming to the school with a Conservative background. The school is not suggesting or encouraging a change in norm for the Orthodox community. I hope that SAR is effective in communicating this message. For some schools this message would be far too nuanced to be effective. The school's community would not be able to distinguish between an accommodation and an attempt to innovate a new norm of Orthodox practice.
Second, let's simplify the halachic issue. There is basis in classical sources for allowing women to wear tefilin. Here, I am making reference to rishonim. However, I beleive that it is generally agreed that there has never existed a practice for women -- in general -- to do so. In other words, the general minhag is for women to not wear tefilin. Now, minhagim can be divided into two categories. Some have no basis in halachah. In other words, they do not reflect a halachic consideration or reinforce a halachic principle. Some of the minhagim in this category are even contrary to established halachic principles. Other minhagim are based in halachah. They reflect halachic considerations and are consistent with halachic principles.
If a minhag is in the former category, I embrace the opinion, articulated by greater scholars than myself, that the minhag may be disregarded or maybe should be suspended. However, if the minhag is is the latter group, it should be upheld.
As other contributors of this discussion have indicated, there is a basis for this minhag and it reflects our more general practice to limit our wearing of tefilin to mandatory circumstances.
I acknowledge the counter-argument, the practice of restricting women from wearing tefifilin, suggests that Orthodox practice discriminates against women. Therefore, in order to present Orthodox practice as consistent with more enlightened contemporary values, we should allow and perhaps encourage women to wear tefifilin. Also, Orthodox women are seeking more meaningful avenues of interaction in the prayer community and this is an opportunity to address that need.
The difficulty with this counter-argument is that accommodating it suggests that the existing norm is discriminatory.
