Evidence of duress can void an agreement, sure. Thing is, there isn't any evidence of duress given in this narrative. I don't see any evidene of 'coercive pressue' nor blackmail. Don't overstate the circumstances. If she wasn't comfortable with the rules given to her, she should have stepped down and let someone else do the speech. Indeed, that's the principled stance.
I disagree. I find ample evidence of coercive pressures applied to this student. In effect, she was informed that if she didn't submit to the school's rules she would be prohibited from speaking thereby impacting the honor she worked so hard for in a negative way. In other words, she was told to say what they wanted her to say or shut up. Nope, no coercion there. Secondly, why should she have to step aside when she earned the honor of being valedictorian honestly? Except for editing for obscenities, she should have been able to speak from her heart. If speaking from her heart entails thanking her God, then so be it. The U.S. Constitution itself begins by thanking God as do the Constitutions of every state as can be seen here. I don't see that as something to be censored. The fact that the school imported an ACLU rep to be on hand speaks volumes to me about the pressures arrayed against this young lady.
As for having a principled stance, that can be argued both ways. One way to look at it is the way you do, i.e. I follow a rule I don't agree with because it is a rule. And yes, that is one principled stance. However, it can be argued that defying a rule that is arbitrarily enforced is also a principled position. Our founding fathers chose the second option. I am thankful they did otherwise we would still be scrapping and bowing to the Queen.