How much should a currently 81 pound 4' 8 12-year-old girl weigh?

According to the Height to Weight ratio for children, the average for a 12-13 year old girl at 58 to 62 inches tall should weigh 85 to 100 lbs. You are at 56 inches (4'8) which is 2 inches shorter, so your 81 lbs is about right, on the low side of the scale. So, you are growing normaly, so far!