Establishment Clause and career day in schools

[Volokh, Eugene]

A correspondent of mine asks:

Can a school restrict participation in school-sponsored "career day"
type events due to concerns about "chuch/state separation"? Our daughter
Lucy had intended to invite her grandmother, a minister, to a first
grade career day to which the school had issued a broad invitation.
Today I was told by the principal that the attendance of a minister
would be a violation of separation. I think that schools are
understandable hyper-cautious about this issue, but do you think that
Peck v. Baldwinsville has any bearing on this? It might be thought that
participation by student's invitees is an aspect of free speech and that
therefore the school is engaged in 'viewpoint discrimination' of the
kind identified in Peck.