You're in 8th grade, not to sound blunt. But, to be honest I didn't start wearing heels until 9th grade mid-year for Christmas. For me, I was 5'3 at the time and felt like I was playing dress up. Obliviously, that's changed and now I love wearing heels when heels are called for.
But, as an 8th grader whose 13 you are still in the awkward stage of being a girl and becoming a woman. Don't grow up to fast, honestly. I say wear flats because you don't wanna try to be bigger emotionally wise than you are.
Also, heels hurt like fuck after a while.
And short is being lovely thing <3 (:
Wear heels, but wear a bit of make up. Not too much, you are only 13 so stay with the natural pastel pink eyeshadows and such. You don't want to come off as strong. At least, not yet. Retain the innocent look and wear heels later.