The T-shirt -- judging by the appendix to the complaint -- seems to be this one:
And as I read the school district's argument (p. 13), the district admits this, arguing that the then-7th-grader's T-shirt violated a school dress code banning any "suggestion of tobacco, drug or alcohol use, sexual promiscuity, profanity, vulgarity, or other inappropriate subject matter."
That strikes me as a clear First Amendment violation under the Supreme Court's decision Tinker v. Des Moines School Dist. (1969). If junior high school students have a constitutional right to wear a black armband to protest the Vietnam War, at least until there's some serious evidence that the armband is likely to cause substantial disruption, they must have an equal right to wear a T-shirt to protest abortions.
And nothing in the school district's motion suggests that the student was ordered to change shirts because of a risk of disruption; the school district apparently thinks that it can just categorically ban any T-shirts that deal with this "inappropriate" "subject matter." One might argue, as Justice Thomas has (and as Justice Black before him had), that Tinker should be overruled; but it's the law, and school districts should comply with it.
The case is discussed in more detail -- though not as to the First Amendment claims -- in T.A. v. McSwain Union Elementary School Dist., 2009 WL 1748793 (June 18). For a similar case, which apparently led to an agreement by the school to allow such T-shirts, see here.