Problem - Student Discipline

Sam Student, an 11th grade student at a public high school in the State of Stone, has been suspended for 5 days.  Sam claims he was suspended because he wrote a letter to the editor published in the local paper that spoke out against the high school's student locker search policy.  The high school principal claims Sam was suspended because he was disruptive in one of his classes and failed to cooperate with his teacher in violation of the school conduct code which prohibits "classroom insubordination."  Shortly after the incident which occurred in Sam's history class, Sam was told to report to the principal’s office.  The principal talked to him about his behavior, told him what his history teacher had said, and asked if he had anything to say for himself.  Sam said he didn’t do anything wrong and the principal should ask other students in the class who would back him up.  The principal then suspended Sam for 5 days without investigating the incident further.

If Sam sues the school as a result of this incident, what constitutional claims can Sam make and what arguments will he make to support those claims?  How will the school defend against those claims?