this was from like four years ago, but i still remember it pretty well. i was eleven or twelve.

i was in this white room, with a white couch and a coffee table in front of the couch. i was wearing a large, white t-shirt, like one of my dad's undershirts. i was not alone in the room- there were a lot of kids in there who looked about my age. all of a sudden, this witch appeared and started chasing us around the room. whenever she caught a child, she reached into the back of their shirt and pulled out their tag and they would disappear. i was one of the last kids left, and when she caught my tag, nothing happened. then i woke up.

can someone tell me what this might have meant?

By tag do you mean "name tag"? Could this have to do with a fear of losing your identity? Considering the age at which you had the dream - around puberty - perhaps it is about changing from a child into an adult.

The colour white is very prominent in the dream; white is a symbol of innocence (childhood) but also of the death of one's identiy.