"Did you already buy a pencil holder for the school?"
Translation:Ĉu vi jam aĉetis krajoningon por la lernejo?
Duo rejects this sentence with "krajonujon" instead of "krajoningon", which surprised me. I'd understood "-ingo" normally implies the container holds a single instance of the content, whereas an "-ujo" would hold multiple. My dictionary gives the example of "cigarujo" as a cigar case. Since a pencil case can normally hold a bunch of pencils, "krajonujo" would seems more appropriate.
The question asked about a pencil holder, not a pencil case.
I think the main difference is that an ingo tends to surround something from four sides and below, with an opening at the top (vase-shaped) while an ujo is a general container: from a bowl to a box.
If you do a Google Images search for "pencil holder", you'll find things that are open at the top - they are more ingoj than ujoj.
You are right, most of the images coming up for "pencil holder" are basically a cup for containing a bunch of pencils in a vertical position, and I was indeed thinking of one of those zippable pouches for carrying pens and pencils in a school bag instead. I stand corrected, have a lingot!