According to the establishment clause, how would displaying a "Happy Holidays" sign in class be viewed?

The establishment clause came into realization that the state and religion cannot be totally separated. According to the Supreme Court, there is a 3-part test in determining if the celebration complies with the Clause,

(1) the activity must be for non-secular purposes.

(2) religion must not be advanced nor inhibited as a primary effect of the activity.

(3) the activity must not result in too must entanglement with the religion.