Some may say that students should study comparative religion or history of religion for their education to be complete. In this case, the exercises being performed do not fit into these categories.
Even though parents had the option to not have their students participate, this still does not prevent the schools activities from violating the Establishment Clause.
In the case of Zorach v. Clauson, the same conditions were not present (activities were not at public spaces with participation by teachers) except for school attendance
History has shown that powerful groups may bring about combining governmental and religious functions or dependency of one group upon the other which would create government support with that group.