You may want to contact your local ACLU for a referral.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
Although teachers and students are free to pray to themselves or otherwise privately celebrate their religious beliefs in the school setting, it is not legal to present religious teachings in a way that suggests they are being officially sanctioned by the school/state. A teacher should never proselytize or otherwise profess that a religion is better than another (or even that being "religious" is good or necessary "to go to Heaven"). A parent can first approach the teacher and ask for such conduct to stop. If not satisfied, a complaint can be filed with the school and ultimately suit filed in court. Good luck.
I have been licensed to practice in the State of Oregon since 1990. I am not offering legal advice regarding your question, only general information regarding the law. You are not my client nor am I your attorney unless we sign a retainer agreement.
Before you go in search of a lawyer or a public interest law office, you should investigate the facts reported to you. As a parent, I had to learn the hard way that 8-year olds are not the most reliable of reporters, and you will need to, too. Any efforts you take to verify the actual events that occurred will stand you in good stead. No attorney or public interest organization will even consider initiating a legal action (or a news report, or even a demand) without a more detailed and confirmed statement of the facts.
Once you have a reliable set of facts, there are a number of important and sufficient steps that you need to step through before you consider recourse by legal action. Start with a meeting with the teacher. If the situation is as your daughter described, you won't have any difficulty in recognizing that fact, and it can be important to elicit a statement from the teacher whose conduct is out of line before the matter gels as a legal case. Then, if the facts are as your daughter has relayed them, move the issue to the administration of the school. If the teacher is out of line with an effort to proselytize religious doctrine, that fact will come as bad news to the school administration and the administrators are entitled to an opportunity to address the situation responsibly. If the school administrators do not resolve the problem (very unlikely), you then take the issue to district administrators -- same reasons.
I very strongly doubt that the matter will need to go that far. There is no controversy within California public schools about the inarguable fact that the schools do not intend to provide religious training -- in any religion. Does an individual employee go off track every once in a while? Of course. Human affairs are inevitably imperfectly conducted by human beings. Does the school administration responsibly and adequately address the situation when that happens? Almost without exception.
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A few background legal concepts should be considered. The First Amendment’s “establishment of religion clause” prohibits the government from enacting a law or sponsoring an activity that has the purpose of advancing religion. Under the endorsement test, the government may not engage in activities that: (1) are excessively entangled with religious institutions; or (2) endorse or disapprove of religion. In 1992, the Court formulated the coercion test when it held unconstitutional the practice of including invocations and benedictions in the forms of “nonsectarian” prayers at public school graduation ceremonies. "At a minimum, the Constitution guarantees that government may not coerce anyone to support or participate in religion or its exercise, or otherwise to act in a way which establishes a state religion or religious faith, or tends to do so." Prayer in public schools implicates both the Establishment and Free Exercise Clauses of the First Amendment. Typically, an attempt to pray at school by either a pupil or teacher is prevented or frustrated by actions or policies of the public school attempting to regulate the school environment. Teachers and students enjoy First Amendment rights within the school environment. This has been true and recognized by courts for over eighty years. Courts balance the individual’s right to freedom of expression of religious speech against the Establishment Clause. Where prayer occurs on school grounds but is private and student initiated, the Establishment Clause is typically not violated. On the one hand, the First Amendment protects private religious expression. When school policy requires prayer or school prayer is public, the U.S. Supreme Court generally finds it unconstitutional; when school prayer is private, consensual, and occurs outside of the classroom, the U.S. Supreme Court generally finds it protected expression.