So, what is it about 6th grade that they suddenly need to only have "healthy snacks?" Be careful here that you're not perpetuating an unhealthy or unattainable body image.
Goldfish are healthy. They're baked, not fried. Veggies, fruit, multigrain crackers and hummus, little bottled waters (be super careful about Capri-Sun and other sugar filled juice drinks), popcorn, low sugar granola bars (stay under 8g per bar).
Occasionally, a less healthy snack is okay. Crispix or Chex Mix. A cookie.
I don't agree with the poster who said they need sandwiches and such. They're going to be going home for dinner. Don't spoil their appetite for a meal with their families by overfeeding them.