IANAL, but in order to win, the plaintiffs have to show that teaching ID does constitute teaching religion. The plaintiff's strategy thus far is to show that ID is fundamentally the same as creationism which has already been determined is a violation of the First Amendment in Edwards v. Aguillard.
Which, of course, is why the ID proponents are trying very hard to discredit Dr. Forrest; she's basically the only expert that can "connect the dots" in her testimony. She's the only expert to which the defense was unwilling to stipulate her qualifications. They worked very hard during the
voir dire to undercut her credibiliity by connecting her with political groups such as the Council for Secular Humanism (to the point where the judge had to slam the attorney rather hard). They raised a completely inappropriate hearsay objection to her entire expert report and testimony (again, drawing a red card from the judge), and are now bending if not breaking the rules regarding
amici briefs in order to get their rebuttal to Forrest into the record without risking having their own theologians undergo a searching cross-examination as Behe did.
And the reason can be summed up in a single question and answer:
Q. Dr. Forrest, is it your view, your opinion, that intelligent design is at its core a philosophical and theological claim?
A. It is my view that at its core intelligent design is a religious belief.
She currently stands in the possibly unenviable position of being the only qualified "expert" on "methdological naturalism and the history
and nature of the intelligent design movement." As such, the statement that "intelligent design is a religious belief" stands essentially unopposed and unrefuted in expert testimony.
ETA: If this case is decided for the plaintiffs, and then is appealed, the situation gets even worse for the defendants, because expert testimony tends to be even more believable when you're just reading the transcripts and not seeing the expert testify on the stand. So you can understand exactly how panic-stricken the DI folks are about being unable to get their counter-arguments to Forrest into the record....