I'm 12 in the 6th grade and my ex said that she would think about takeing me back but she said that in dec. what do I do

You forget about her and move on. You're 12, there will be many more exes in your future. At this age dating isn't serious, so just move on and find someone else to hang out with. :)

First you're still very very young, age 12 is too early for some serious relationship and of course this applies to both you and your ex, she probably think the same about this to not take you too seriously . You can find someone else to hang out with, for now just take the experience from your pasrt relationship with your ex and use it for your next relationship , you still have many good thing ahead waiting for you so you shouldn't stuck wanting your ex back.

Move on to the next girl.

This is why 12 yr olds shouldn't be dating. You are too young to have an ex. Move on!