Need advice... what to say to your "friend" that stole your date.?

What a doozy. . . My personal opinion? Sounds like Sonja isn't being much of a friend at all and if I can remember correctly, eighth grade can be all about life-ending decisions. The angst you are feeling right now won't last and likely won't matter in a few years . . . But since it matters right now the best thing that I can say is for you to sit Sonja down and tell it to her straight. Explain to her without any holds barred that you and Eric and already agreed to go together and that the way she is acting is running a little close to the lines of selfishness. If she still won't listen, then you should decide whether or not a friend like that is worth having -- and if one dance in the long run of things is really worth freaking out over.