A good snack option is a smoothie in a thermos. We aren't school age yet, but this is my preferred on-the-go meal for DS. It's no more than dumping a 1:1 ratio of frozen fruit and almond (or other) milk in the blender with a scoop of whey protein. If I'm feeling fancy I add a Tbsp of a nut butter or ground flax seeds. I could see a small smoothie working well at snack time. If you want the smoothie to digest more slowly, make it a casein protein or whey-casein blend.
A great option for healthy fats, if you don't already do this, is a pharmaceutical grade fish oil supplement with breakfast. You can buy either enteric coated capsules or flavoured oils, which are surprisingly kid-friendly.
If you're feeling adventurous, you could offer standard "dinner" meals for breakfast to increase protein intake in the morning. My colleagues used to laugh when I are tilapia with steamed veggies and beans at 9am, but that sort of meal gave me the greatest mental clarity early in the day.
