About family trees: isn’t it exactly the fact that people’s families are so different that makes such a project interesting in the first place? This might depend on the teacher - but if a teacher introduced the assignment by briefly talking about different kinds of families and gave specific suggestions about how people could draw in two sets of parents, deceased relatives etc., would you feel differently about this?
I think it would still be a really bad idea and that the reasons I described in my original post still apply.
No matter how a teacher frames this, it’s a really sensitive subject.
It’s great if teachers are sending the message that there are a lot of different kinds of families. They should send that message, and one way to do that is to assign books that show lots of different kinds of families and cultures.
It’s not good to use students as object lessons, though. Particularly if they’re dealing with something painful or socially stigmatized. 
For instance:
  • A kid who’s been in foster care since they were three and doesn’t know who their parents are shouldn’t have to decide between lying and announcing that to their whole class.
  • A kid whose mother just died might not want to talk about that.
  • A kid who has two fathers might be afraid to tell other students that, particularly if many of them are members of a faith that stigmatizes homosexuality
  • A kid who doesn’t have ready words to describe their family situation might not feel comfortable discussing this with you. They shouldn’t have to choose between lying and having a scary conversation about something personal.