The are very few people who are really anti-religion, or stand in opposition of having someone else participate in religion. The First Amendment, in effect, says that no governmental authority, including school systems, may authorize, require or restrict the freedom of religion. That is a basic building block of the Constitution, which established and regulates our great nation. Among those who first came to America, and those who put together the documents for our government, were believers and nonbelievers, those seeking religion and those escaping religion. The consensus and wisdom led to this portion of the First Amendment.
Most school systems derive some or all of their revenue from real-estate taxation.
This means that every person who lives in our great country, who owns real estate, will be forced to pay money that will be utilized to develop and maintain public school systems. They must pay those taxes whether they approve of the school system or not. If they are required to pay those taxes, and then those taxes are used to promote religion, that becomes unconstitutional. Any student, at any time, can, to himself or herself, recite prayers, seek the guidance of the Lord, express appreciation or meditate. Any group of students can do likewise.