Can a principal permit or disallow a walkout to occur?

No, walkouts are not at a principal's discretion. As stated in Howard County Board of Education Policy 9020 Students’ Rights and Responsibilities, “Students have the right to assemble. Students have the right to freely choose the subject and form of a peaceful demonstration. Schools may set reasonable limits as to the time, place, and way students and student groups may assemble if those restrictions advance legitimate educational goals.” 
 
School administrators will disallow students from engaging in a walkout only when the purpose of the walkout violates Board of Education policies and HCPSS procedures.