My experience is that what the child is willing/able to drink is more important than how perfect the ingredient/nutrient list is for prepackaged smoothies/shakes.
My son is a 70lb, 12y 10month 7th grader.
We have made the most progress by almost giving up on school lunch and giving him snacks that provide as many calories as we can get him to take (yes, it violates the school policies.) We focus on getting him awesome breakfast, a light lunch instead of snack when he first gets home, and a really healthy dinner (with high-cal options that the adults don’t eat).