The Freedom of Speech is many times misunderstood. It is a constitutional proscription that prohibits the government from stopping, preventing or otherwise censoring speech on public grounds. Having said that, Freedom of Speech is NOT absolute. The Supreme Court has ruled many times that free speech has it's limitation.
A public school is on "public grounds" so it would appear natural that Free Speech would be unilaterally tolerated. Legally, it is not that clear cut. When and if this case goes to a Federal court, the issue will not about abortion, but rather, whether the school violated the Establishment Clause by allowing, and thereby sanctioning, a pro-life student group on their campus.