Hm... I don't know the technicalities of it all, but I can make guesses.
If you're a school-based team (IE, one school and it's a recognized/supported school activity), I'd guess that your school owns the bot. If you're an independent team (random people who came together), the team in general owns it. If you're a multi-school team, it's all muddled.
I don't think your sponsor owns the robot unless that was something they requested when they agreed to sponsor you.
But... I could be wrong. I'm a high shooler, not a lawyer.
--EDIT--
We have our robot is a barn one of the families own.