The teacher's attempt to force the student to recite the Pledge of Allegiance violates the constitutional doctrine stated by the Supreme Court in the 1943 ruling West Virginia Board of Education v. Barnette. In Barnette, the Court overruled prior caselaw and held that forcing students to salute the flag and recite the Pledge of Allegiance "transcends constitutional limitations on [the school officials'] power and invades the sphere of intellect and spirit which it is the purpose of the First Amendment to our Constitution to reserve from all official control" (emphasis added).
The teacher's conduct also violates state policy. A Maryland student handbook states that: "You cannot be required to say a pledge, sing an anthem, or take part in patriotic exercises. No one will be permitted to intentionally embarrass you if you choose not to participate" (emphasis added). The other students in the class mocked the defiant student as police escorted her away from the classroom.
On the first day that the student refused to recite the Pledge of Allegiance, the teacher yelled at her. Now, the teacher must apologize -- thanks to a settlement negotiated by the ACLU.