There are lots of different types of Ziploc bags, but the type that work the best for pencil cases are the 1-quart (0.95 L) bags made from thicker plastic. It would be even better if the bag has a slider on the top. This will make it more zipper-like and easier to use. Avoid using a regular, sandwich-sized bag, as it may not be big enough for your pens and pencils.

Summary:
Get a 1-quart (0.95 L) Ziploc bag.