At some point, the cheerleading squad at Kountze High School in Texas began inscribing the banners at football games with Bible verses such as "I can do all things through Christ which strengthens me!" To the cheerleaders' way of thinking, the banners are a way of exercising their freedom of speech, and a judge has at least tentatively -- and wrongly -- agreed.
Certainly, students are entitled to express their personal viewpoints in many school-related settings. But a cheerleading squad is a school-sponsored organization that is supposed to be equally welcoming to all students of all beliefs; the cheerleaders wear school uniforms and perform at official school functions representing their school.
If the word "school" seems to appear frequently in the previous sentence, it's because there's no way of getting around the impression that the banners have the school's seal of approval.