That's a no-brainer. Try to comprehend Paradise Lost (and all the other glorious things John Milton wrote), Edmund Spenser's The Faerie Queene, the morality play Everyman, or the Faust legend without the Bible and you are lost. The professor in my Lit class had to give a summary of Christian (Catholic and Protestant) theology in class so that the students (most of whom had no clue) would have a better understanding of the texts we were reading.
As a HS English teacher, I had the same situation in my British lit. classes.
In my high school senior year '63, we studied "The Old Man and the Sea" to get an "A" the final essay had to discuss the main character and his "Christ like" image. Even though Hemmingway said he did not deliberately put a Christ image in the story. We also had a chaplin give grace at our Seniro breakfast. We turned out alright.
I believe schools need to get some courage and realize that part of the reason teachers have tenure is so that they can be a little controversial and not lose their jobs. No one wants to see the schools or local governments begin to "establish a religion", but we would like them to teach.