How can I tell which products in the store are limited to certain grades?
When you are a store manager or admin browsing the store, any product that is limited to grades your own students are not in shows a small pulsing dot in the top right corner of the product. Hover over the dot to see which grades the product is for. This is the same dot used to mark products that are in Preview; if a product is both in preview and limited to certain grades, hovering over the dot explains both.
You can also open the product from Store/Manage Products. Its details page has a Grades section listing the selected grades, or All grades if the product is available to everyone.