Translation:Did you already buy a pencil holder for the school?
If I understand correctly, an INGO can only be a sheath for an individual object, while an UJO is a container for an assortment of related objects. In this case, would a KRAJONINGO be a sheath for an individual pencil? That seems like an odd thing to purchase for school... I was expecting KRAJONUJO. What am I missing here?
I too was surprised to see this in the course. This was in January. I have come to the conclusion that you are correct. The problem is that -ing- is rather uncommon, so the course creators didn't have a lot of examples to choose from. The general idea is that an ingo holds a single straight object, usually with part of that object sticking out.
Since "ombreligo" was mentioned elsewhere, that is a word, but it's an "umbrella case" for a single umbrella. The thing that you stick your drippy umbrella in, since it holds multiple umbrellas, is an ombrelujo.
I don't think it's the "for an individual object" property so much as the "sheath" shape -- for example, an umbrella stand might be a "pluvombrelingo" since it is usually tall, narrow, and open at the top even if it can hold three or four umbrellas.
An "ujo", I think, tends to either have a wide opening (like a bowl) or to be closed (like a case or box).
My original lessons on this topic, and I still recall them lo these many years later, included bits like Maningo = glove, Brakingo being an acceptable substitute for sleeve, and Tondilingo = a scissors case. Ingo was always designated as something close fitting. Now, that may have changed with usage out in the real world of people actually getting to use Esperanto together but one's original lessons still stick. To me a krajoningo would be something that holds and protects one pencil at a time. A "pencil holder," usually, in English at least, a can or some such adapted for such use, would be Krajonujo. And a case for carrying several writing implements at one time while traveling (as in "from class to class") would be Krajonportilo or Plumskatolo.
The Pluvombrelingo by this reasoning, is the cloth or leather case one puts on the umbrella while out but when it's not raining.
So, yes, I'm with Hakanto on this one.
Regardless, this sort of phrasing is in common usage.
One also hears: "Did ja already get…" which leaves open the question of whether that act of acquiring something was done legally, or not. If dictionaries hadn't already been invented ja (pron: ĵa) might have become English for vi.