How much should a 5th grade girl weigh?

"Should" is a bit of a disclaimer here. I'd assume you're referring to what is a healthy weight range for a 5th-grade girl. Honestly, it depends on several factors, such as your height, body type, even your body's maturity. Race also plays a role in this, as it determines your genetic markers and can even influence your average weight. (i.e. Asians tend to weight less than Caucasians, etc.)

Let's say an average 5th-grade girl is roughly 5'2", or 157 cm. A healthy average range of weight could be anywhere between 103 lbs. - 139 lbs., or 47-63 kg.

Again, this depends on your height and body type. Definitely ask your doctor for better clarification on this topic.