Thankfulness is a theology in microcosm, according to Albert Mohler. He says the practice of giving thanks is, "key to understanding what we really believe about God, ourselves, and the world we experience." So, schools say a lot when they celebrate Thanksgiving.
A lot of schools teach the history of Thanksgiving, telling a type of neutered story of pilgrims and indians celebrating diversity by sharing a meal. Few schools give details to the horrendous plight of the pilgrims, how many of them died as disease plagued their colony. The entire settlement would've failed without, "a special instrument sent of God," as William Bradford wrote of Squanto. The first Thanksgiving was an honoring of the providence of God for a fledgling settlement.
Nearly 200 years later George Washington bolstered the new nation with a Thanksgiving proclamation to, "acknowledge the providence of Almighty God, to obey His will, to be grateful for His benefits, and humbly to implore His protection and favor." The United States was settled with a group who understood the providence of God, and the nation formed as leaders routinely acknowledged God, praying for protection and favor.
A quick Google search shows the United States to be a country that positions itself under the will of God. Numerous leaders followed Washington's example. Secular schools are reworking the history surrounding the nation, failing to teach students the history and also the practice of giving thanks and praying for providence. This is something Christian schools practice daily, often through devotion time or prayers before class. These tiny habits change lives by teaching students that all knowledge rests under the banner of Christ.
In Duet. 6 the nation of Israel is found about to enter a challenging land. God gives the nation start-up a guide to remind them of His protecting hand. As the United States formed, leaders made similar practices, celebrating providence and imploring God for wisdom during difficult times through thanksgiving pronouncements and calls to prayer. Many secular schools omit these parts of history, and many cannot practice prayer or a call to giving thanks. They cannot acknowledge God. To not acknowledge God, namely giving Him thanks, is devasting to mankind.
Paul warns the church in Rome that failing to acknowledge God, noting specifically giving God thanks, causes men to become futile, with darkened hearts and foolish thinking. This is what many state schools are doing to students in teaching them a hallow practice of giving thanks without God.
State schools turn to Stephen Gould and Carl Sagan, like the two are wise uncles sitting in front a bowl of cranberries. Gould and Sagan praise the universe for providentially crafting an environment for mankind to wallow from. Thanksgiving avoids God for men like Gould and Sagan. They instead turn to the great feats of knowledge that man has achieved in understanding the universe. Sagan is known for praising us, almost illiciting a thanksgiving proclamation for our own wisdom and courage in surviving in the universe.
These two are bedrocks of secular education. But, the pinnacle of Thanksgiving for the public school may rest with a grandfather's thanksgiving sermon from 1897. Robert Ingersoll wrote a winsome piece, opening thanksgiving by bemoaning the impact of the church in history. He focuses his thankful address away from God. Ingersoll Praises a hoard of men, ranging from Robert Burns to William Shakespere. He thanks dead men who he thinks progressed society. This is a common practice in secular education, teaching kids to love the work of men. This practice forces children to exchange the image of the immortal God for mortal man. Paul writes about this.
In the end, all schools teach students to be thankful to something. Students throw their caps at graduation appreciating either the work of men, or in celebration of the work of God. Christian schools change lives because they teach students thankfulness rooted in God. Secular schools teach students to revere man, which causes students to become foolish in their thinking, according to Paul.