I really don't understand what the purpose of doing "family tree" assignments in school. If you are ever to do one, it would make more sense to do it at home...with your family as a "let me tell you about where I'm from" type of thing not a "hey tell us which family members are related to who" thing. I don't even get the goal it serves for biological kids, yet it is a very common assignment for young children. For those of us who are adopted or foster children, it's a giant slap in the face and constant reminder that we aren't really a part of our family.
I remember the first time I got one. It was from Sunday school. Why were we doing family trees in Sunday School? Who knows? I spent days worried about it. I didn't know if I was supposed to put my family or my family. Was I supposed to do the people who adopted me? Because we're not blood-related and a family tree is about lineage...we have no lineage. I could fill it out for them, but then I would not be on the tree because we do not share a lineage. Or was I supposed to do my birth family that I know nothing about? At one point, I tried to figure out how I could include both but was quickly reminded that all I knew about my birth family was my birth mom's first name. I filled in the spot that was for me and never completed it. Because how can I? I don't have a family tree. The only part of the tree that can be completed is my square.
I've read so many stories from parents mentioning their adoptee being sent home with a family tree type assignment and wondering what to do about it. I don't have an answer for that question because my advice is to just not do it. It's already a pointless assignment and no kid cares about the history of their lineage in the first place. Stop assigning these alienating assignments. If you're going to make family related assignments, make it about the child or allow them the creativity to decide what direction to take. You don't know who in your class (or maybe you do, in which case makes it even worse) is adopted. Hell even if they aren't adopted, there are a lot of kids who have shitty families and telling them to dive into their family histories can bring up a lot of trauma that didn't need to be brought up for the sake of a stupid crayon sponsored family tree.