In the case in the text, Olmstead v$.$ Saint Paul Public Schools, the court held that:

Olmstead's resignation was revocable because when he resigned he was under duress.

Olmstead was under duress when the District threated to file termination charges against Olmstead when it had no intention or grounds to do so$.$ the District illegally promised to not report Olmstead if he resigned.

Olmstead could not demonstrate that his free will was overcome.