“The banner was unfurled directly across the street from the school, and pointed in the direction of the school and students, therefore it was in the Court’s opinion, a disruption of the school and it’s discipline.” Why do schools have extra constitutional protection from “disruption”. Every other institution, even private homes and businesses have to put up with disruptive protests where banners are unfurled directly across the street from them, if protesters feel so inclined. What makes schools immune to protest and able to suppress freedom of speech even off of school grounds?
Not being a lawyer, it's likely becuase they are held responsible for the welfare of the students in their care and while acting in absense of parents. If you really need an answer, I'd suggest writing to a constitutional lawyer for an greater and more accurate reply to your query.
I'd also suggest you take 1/2 an hour and read the ruling itself, which is in depth.