Depends. If school policy is no politcal message t-shirts, then it's OK constitutionally.
If not, then the Principal should be fired.
A political message involves suggesting how people should vote, the shirts didn’t do this.
I think the issue here is that the consequences kept changing. If the penalty for not following the dress code about political speech is two days suspension, then they should have received it. HOWEVER, the addition of missing senior prom, etc. cannot and should not be added on.