This reminds me of when I was a child, in elementary school. Every year we had a little cultural fair/festival where we'd be required to bring in foods from one or more of our ethnicities, in addition to giving a presentation to the class about our ancestors and the countries they came from.
At that point in time, I didn't know who my mother's parents were as she was adopted, and people were doubting my biological father was my biological father (turns out he was my biological father, thanks for doing a DNA test before you left this life, old man!). So I used my adoptive father's ancestry instead for the first year I had to do this, and I not only failed that assignment, but I wasn't allowed to participate in the festival/fair, eat any of the foods, play any of the games, but I had to watch everyone else enjoy it. And my mom was sick with cancer at the time so little G. didn't bring it up because I didn't want to worry her... if she had known her first grader was put through that, she would have raised hell.
Long story short: instead of a family tree, how about an "immediate family web"? Instead of a culture fair where you look up your own culture, a culture fair where you have to present on a culture you have no ancestors from?
Here's hoping if any teachers run past this, they'll think twice about the family tree thing.