How should I handle a school family-tree task when my daughter doesn't want to explain her adoption?

The worksheet asked for a baby photograph and a family tree reaching back three generations. My ten-year-old drew a box around the first instruction and wrote, "I don't have that photo."

She joined our family at four. We have photographs from then, but no baby picture. She knows her history and talks about it with us when she chooses. She doesn't want to explain it during a class presentation.

I suggested using a four-year-old photograph and telling the class why. She said, "Why do they need the why?" I had assumed openness was the best response. She saw an ordinary homework task becoming a request for personal information.

The teacher hasn't been contacted yet. I don't know whether the instructions are flexible or whether she expects every family to use the same format. What would you ask for, and how much would you share with the teacher so the task can change without making my daughter the example everyone discusses?