Most WCAG criteria are applicable to multiple types of disabilities but the guidelines themselves are not grouped by disability. So if you're looking for something specifically for hearing impaired, it's hard to search the guidelines that way. Yes, there's information about captioning, but captioning benefits more than hearing impaired. How often do people use captions on a TV in a bar, gym, or airport? There are many uses of captions beyond hearing impairments which is why the guidelines don't limit it to specific disabilities.
However, that being said, there are a few guidelines that certainly benefit people with hearing impairments.
1.2.1 is usually fulfilled by having a transcript of the audio. But it sounds like in the comment section of your OP that these could be young kids with a minimal reading ability. That's difficult to solve. From a legal compliance perspective, having a transcript would be sufficient but then you'll have to work with educators to get their ideas on how to convey a transcript to a non-reading person.
Note that a transcript in digital format, not just paper, is beneficial to lots of people. Searching a digital transcript is pretty easy. Even a basic editor can search a document.
The transcript can also be translated to braille by using a refreshable braille device (no work is needed on your end). There might be a deaf-blind student that can't see a transcript but if it's digital, they can read the text as braille, although this again assumes a certain reading level.
1.2.2 talks about captions, which you mentioned in your OP. As far as the cost to develop, there are a lot of companies out there with affordable captioning. I'm sure you didn't imply this in your question but if the reason for not doing captioning is because of cost, what price is the boundary between being affordable and expensive? Or another way to look at it, how low does the price have to be to create an inclusive product for all students. Or at what price does it go above that justifies excluding certain students?
There is a "reading level" guideline, 3.1.5 Reading Level, but it's more about keeping the reading level down to about an 8th grade level for adults and doesn't address alternative ways to read for young kids. That sounds like a great PhD topic.
If you can satisfy both 1.2.1 and 1.2.2, that's a great selling point for your product.