The whole idea of requiring a "secular purpose" is what is wrong with the Lemon Test. It seeks to determine the religious motives of the lawmakers rather than simply consider the actual effect of the law.
In that sense laws against stealing could be overturned if the motives of the legislators were to adopt portions of the Ten Commandments into law. Further, the purpose prong of the Lemon Test creates the very "excessive entanglement of religion and government" prohibited by the third prong of the test, only this time it is the Courts and religion that have become entangled - with courts questioning the religions motives of the lawmakers (or school districts) in enacting the laws or regulations.
The Lemon Test has given us contradictory and absurd results and I believe that it's time is up. It is a stupid way to analyze first amendment questions. I believe that if this case is appealed, that the Supreme Court will abandon this dinosaur.
Bingo. If the ACLU appeals, the USSC will grent Cert if only to toss Lemon onto the ash heap of history.