They have the right to keep her off school grounds.
But would they do the same thing if it were subhuman savages chanting their pseudo-religious slogans?
To the extent they allow other parents on the grounds, then she should be allowed on the grounds. To the extent they allow other parents to talk while on grounds, then she should be allowed to talk.
However, I see no reason to allow the government to ban people from people-owned property. Security checks? Sure.
But, to refuse parents the right to monitor or be involved in their child’s education is tyranny.