I wouldn't worry too much about size. My youngest will enter 6th grade this fall. She turns 11 next month and she is 4 ft 3.5 in, 51 lbs - I think that is on the small side. Typically is 1st percentile for BMI. Middle kid was similar size at that age but she starting growing around the 11th birthday and is now average height, sort of average weight (5'5" and 105). I didn't finish growing until I turned 18.
Sure, there are times my kids wished they were early rather than late growers, but they will be fine. I don't listen to the pediatricians. We all had a laugh when they sent middle kid a letter saying they (supposedly respected CHOP) wanted to include her in a study of children who will be adults of short stature - for females that is under 4'10". I know my kids won't be tall, but just because a kid is short at 10 or 11 it isn't the end of the world. Hopefully 6th grade will meet your kiddo's educational needs.
