Magiver, do you have a clue how to construe the Establishment Clause and the Free Exercise Clause together? Because there are reasonable grounds for holding a different opinion than the Supremes on issues at hand – but the above ain’t one of them.
It’s quite legitimate for a Valedictorian to thank God for his or her success in school in private, or at a voluntary public gathering that can include everybody graduating from school, their parents, the school board, and all nine members of the Supreme Court for that matter, or in church, or on the school grounds whenever reasonable school regulations on student behavior do not prohibit it.
What he or she may not do is, at a graduation ceremony, attendance at which is mandatory for a student to graduate, where he or she has the privilege of making a Valedictorian speech, use the occasion to evangelize his or her fellow graduates – because he or she is coopting the authority of government – remember that attendance at the graduation ceremony is mandatory to receive one’s diploma – to press his or her religious views.
You may want to review Cole v. Oroville Union High School. My distinct impression is that a student who acknowledged his or her fellow students’ diversity of beliefs (and disbelief) and in the course of acknowledging influences on his/her life included a reference to God’s Providence and his/her own gratefulness for His blessing, without extensive discourse of evangelistic nature, would not violate the Establishment clause but would instead be exercising his/her own Free Exercise clause rights. This is illustrated by the discussion about Cole’s and Niemeyer’s speeches in the constitutional context in the cited review of the case. (Sorry to use “About Atheism” – as a sincere and freedom-loving Christian, I have to admit that they are one of the few unbiased sources around on the background behind what the Court actually said!)
The point, if it’s not clear, is whether anyone can use government authority to compel someone else to listen to what he/she has to say about religious belief – they cannot – not about whether or not anyone has the right to speak their mind – they do. The one thing they may not do is use the power of any part of government whatsoever to compel themselves a captive audience.
