Back when I was in public elementary school (a long time ago), our teachers would lead us in a brief nondenominational prayer before lunch. That was a good thing!
The key word there is “nondenominational”. Is a poster listing the Ten Commandments nondenominational? I think not. Therefore, I am very reluctantly against such a poster in a public school.
I think you mean, "non-sectarian" rather than "non-denominational". Denominations are generally understood to be separate groups under the same general belief system. Thus, for example, Baptists, Presbyterians, and Methodists are denominations of Christianity. However Judaism or Hinduism would not be a denomination of anything but entirely different religions, or "sects" (not in the popular modern usage as referring to a cult or esoteric religion)