Back when I was a student, alternative text wasn't really a thing. If I was lucky, I'd get a PDF from the publisher. If not, I'd have to scan and OCR the book myself. Just the fact that you exist, and have a set of guidelines to follow, is a huge step forward.

As for the text itself...it depends what the image is used for. If it's a chart/graph, how will it be used in the classroom? Sometimes, you might need to provide every single datapoint, and sometimes, that might be overkill. The same really applies to other images as well: if they're just to ad flavor to the book, they don't need as much description as if they're integral to the text. I guess the question to ask yourself is: why did the author of the book include this image, and is my description serving the same purpose as looking at the original image?

I know I experience frustration on some government websites where the description isn't in sync with the purpose of the image. A stock photo of, say, a man standing next to a tractor really doesn't need a 2 paragraph description. Especially when the article is about something unrelated like the environmental impact of farming, and the article never mentions men or tractors at all. Information about the man's overalls and hat, and what kind of tractor it is, just distracts from the article, and thus takes away from the author's original purpose, rather than adding anything useful.

Totally blind Accessibility SME here. I helped author the U.S. Social Security Administrations guide to alternative text. It is a quite comprehensive look at the art of providing clear and concise alternative text, plus it prominently features my old guidedog Pastel. Give it a look: https://www.ssa.gov/accessibility/files/SSA_Alternative_Text_Guide.pdf