underweight = injuries?

Your BMI of under 17 is underweight, so it’s possible and even likely a cause of your frequent injuries. You probably lack muscle mass and strength. Not eating enough can result in soft tissue injuries, not just stress fractures, because the body doesn’t have protein and/or total calories for tissue repair.

I think a lot of runners, especially younger ones, underestimate how much they actually need to eat, especially how much fat and protein. You don't have to have an eating disorder to be underfueled. You might want to see a dietician for help.

There's also more to weight (and lack of) than body fat, like muscles and bones. You can prevent injuries by building strength, but of course you'd need even more calories and protein for that.

One of the current high school greats is 101lbs at 5’4. Her name is Ellie Shea:

Underweight = faster but more prone to injuries

I wouldn't compare one high school age runner to another. Better to look at older runners who have been consistent throughout a number of years.

The point is that it is not valid to conclude outright that the OP is too thin, and that her thinness will continually lead to injuries (particularly when she says she is eating and doesn’t have an eating disorder.) Looking at older runners is not much help, since runners will typically grow in size as they continue to mature. Bottom line is yes, she should get some advisement on her dietary needs.

No. Overweight causes more injuries.

Just eat baby

You are not underweight. There is nothing wrong with you, if you are eating properly. First, get a PT to check your biomechanics and check for a leg length discrepancy. Add yoga to your routine, especially if you are sitting all day at school. If your body is out of wack, it will lead to injuries.

If your nutrition is okay, perhaps you are training too hard? Maybe the prescribed training does not work for you. Sit down with your coach and come up with a different, perhaps less aggressive training plan.