Science teaches evidence-based and reproducible knowledge. Religious belief and pseudo-science (such as creationism) should not be allowed to be taught in science classes in publicly funded K-12.
First amendment protects freedom of speech and religion.
However Public school systems are funded by the US government via tax-payer money. In doing so making public school system an extention of the government.
Under the requisite separation of church and state. Thus technically schools can't be preaching other than in social studies or religious studies.