It's been pretty well established for some time now that public schools, being a part of the government, are to be neutral in matters of religion.
No, they are not a legislative body and no, they are not enacting laws. They still are under the completely logical and legal precedent that had established a separation of religion and government.
Because the US has no federally recognized official religion, it logically follows that the US cannot give preference to one religion over any other. This applies not just to schools and congress but also to courts, the DMV, the SS office, the military, etc.
So there is absolutely no reason why a picture of Jesus should be hanging in a public school unless that school has no problem hanging up a picture of Satan, Buddha, L. Ron Hubbard, Joseph Smith, or Moses. And mind you, this is still ignoring literally thousands of other religions that are recognized just like Christianity. If the school truly wanted to be impartial, they'd have more pictures of religious figures than there are students in the year book.