Many traditional sports slogans focus on individual worth and collective achievement. That's also a focus of secular humanism, which many atheists tout as an alternative value system to religious faith. Should Christians argue that they've been “attacked” if banners declare “hustle and heart set us apart” rather than citing God as the source of success?
The First Amendment exists not to suppress expression of faith but to maintain state neutrality toward competing value systems. School officials aren't forcing students to engage in religious expression. So what's the problem?