My son is a college runner, and he competitively since 7th grade. He is "naturally" thin (I know, because he has an identical twin brother who doesn't run and he's quite slender: runner, 120-125#, non-runner. 130-135#, both 5'9") and it was very hard for him to maintain his weight during racing season during grades 8-11. I know it's different for boys, but here's an idea of his diet during high school.
Water: carry a watter bottle all the time, refill it from the water fountain at school, etc.
Breakfast: anythng that can be eaten quickly! Two packets of instant oatmeal, granola bars, whatever.
Lunch from the cafeteria: bagel, yogurt, canned or fresh fruit, milk
Snack (often had an hour or so before practice): banana or granola bar
Dinner: usually some kind of meat and some kind of grain.
Bedtime snack: apple, peanut butter, cheese.

The bedtime snack was especially important, because he often did not have time for a big breakfast or did not want to eat a lot before a Saturday morning race. So with some high-quality protein before bed, he wouldn't be starving on race morning.

Another thing is, after cross country season and before winter training, he just would veg out in front of the TV for a week or two and eat. He was able to gain back the weight he lost during racing season-- and even grow!

Don't worry about the gatorade and protein bars - they belong in the trash, not someones stomach.

Being skinny isn't necessarily unhealthy. Everyone has their own body shape, and changing it can be difficult. As long as she seems healthy and doesn't have performance problems in races, she probably is healthy.

If she starts losing weight, or seems to not be running well (ie. loss of power, bonking during races, getting sick often) then you can worry.