I attend a public school in California, there is a voluntary field trip day, once a year, where you go to a church.
In the church, they put on a Disney show, with nothing religious about it. The subtext is "make churches happy experiences to try to get converts".
Are schools and churches allowed to collaborate to get students into the church? Is this illegal, and if so, by what law?
This seems slightly fishy, but I can't think of anything that would make this illegal.
It would be unconstitutional for there to be a law against (public) schools having a field trip to a church or other religiously-centered building. The First Amendment requires government action to be neutral with respect to religion. This means that a public school can neither promote nor condemn religion. A field trip where the venue was a church is not per se the promotion of religion in general or a particular religion. A pattern of activity, and associated facts, could, however, establish that a seemingly innocent event is in fact religious proselytizing. If for example the teacher actually says "I want to make churches be a happy experience, so that students would be inclined to convert", that would cross the line from neutrality to advocacy. If there were many possible venues for such field trips and yet only the church is chosen, that might be evidence of non-neutrality. It really depends on the totality of facts. If the church has the only stage in town, that would be a perfectly reasonable basis for repeated trips to that church.
I would disagree with @user6726 somewhat on the application of the legal standard to these facts even though I don't think that we disagree much on the applicable legal standard.
If a church venue is used simply because it has a large auditorium space and the program has no religious content, the mere fact that the program is conducted in a church does not violate the establishment clause. The program has to promote or condemn religion to constitute an establishment clause violation. While it is easy to imagine a field trip to a church that does promote religion, the field trip in the question shows no sign of having that purpose.
A speculation that there is an improper motive to make students comfortable with a church setting to encourage them to convert, as the question suggests, without any solid evidence to back up that inference, is not likely to prevail in court.
Along the same lines, a field trip to a baseball stadium does not violate the establishment clause of the First Amendment simply because on Sundays mornings, a local megachurch conducts its worship services there.
