Since my son is almost 2 we are just starting this phase, but I know many other poly parents who've shared their techniques with me. It seems to depend on the parent's comfort, the child's comfort, and the local environment's tolerance level. This is a complex interplay of factors.
While we plan to clearly indicate that our son has Brett and I for parents and the others in our household are "aunts" and "uncle" type significant others in his life, I know several families where all the adults involved (3-4) are presented as having parental authority over the child to the school and other public associations--there's an article about one such family in our upcoming issue of Loving More Magazine.
One parent I know told her child from early on that if he shared about their different kind of family life he might lose friends and alienate people--then she left him to choose to disclose what info he cared to--lost one friend, kept most. One larger group marriage I know selected 2 adults to be the parents in all public situations (they were not the bio parents) for their teenager when she entered a new school.
My prejudice is to share only what feels natural and relevant based on a "need to know" basis--this means not much to officials and a lot to close friends/family.
On the Geraldo show we did recently (still not aired) he hit me up a lot about how my kid would be teased by classmates and confused by our lifestyle (none of us are confused, so I don't think Zeke will be either). It seems everyone is teased in school about something--I'm not going to change my values to avoid teasing! Like should I change that I'm a vegetarian sometimes vegan, not Christian, a libertarian leaning democrat, supportive of gay rights, etc, etc--all those things that may or may not be popular in my neighborhood?