I'm afriad it is you, lunatic, who are mistaken.
Read the case. The school district accomodated him by allowing him to leave the classroom if he so chose. It's in the first couple of paragraphs of the case. If they locked him in the classroom, then they didn't need any Supreme Court case to determine the illegality of the law. He could have simply sued for false imprisonment.
William Murray claimed when he tried to leave the classroom after being "excused" from the mandatory prayer, that the principal locked the door and refused to let him leave. Is it true? I don't know, I wasn't there. He also claimed to have been physically abused by his fellow classmates when he tried to leave. I don't know if this is true either, but it would not surprise me. You and I both remember how kids and peer pressure can be in high school.
And your solution, "he could have simply sued for false imprisonment," does not address the underlying unconstitutional promotion of religion by a government institution. Christians have been pissed off for years that they no longer have influence over public schools, and this case with the drama teacher in Missouri is an example of their attempts at subtle influence over school policies.