Answer: Under McKinney-Vento, schools are required to review and revise policies and practices that are barriers to enrollment. Enrollment is defined as attending classes and participating fully in school activities. An overnight field trip with a class sounds like a school activity. Therefore, the school must find a way for the unaccompanied homeless youth to participate in the school activity. This could include signature by the youth herself, the liaison, or a caretaker – very similar methods to how unaccompanied youth enroll in school without parents, and over parental objections.
One concern: Since the student is still a minor, and the parent is specifically saying she does NOT want the student to go on this field trip, it definitely opens the district up to liability. McKinney-Vento acts somewhat as a shield, but it is a risk if there were to be an accident or other problem on the trip. It may be a good idea to talk to the school district lawyer about it.
Follow up note: This liaison spoke with the district lawyer, who agreed that it was ok for the student to go on the field trip, since she is on her own.