Her response to the protesters on Friday could encourage more.

“We will unleash our activists in a way that I don’t think any secretary of education has ever experienced,” said Heidi Hess, a campaign manager for Credo, a mobile phone company with a liberal activist arm. “If she holds field hearings, we will make sure we pack them with activists. If she travels for meetings or if she visits schools, we will confront her with protesters and have people lined up to ask her questions.”

As word spread that Ms. DeVos, a wealthy Republican donor with almost no experience in public education, had been blocked from entering the school, Arne Duncan, a former education secretary under President Barack Obama, weighed in.