I think a parent visiting schools can add the library to the list of factors to consider.
On the other hand, it depends on the school. Don't assume large library = good, and small library = bad.
Cushing Academy famously gave away much of its library. This blog post links to newspaper reports of the decision, and its consequences: Out of the Jungle: Update on Cushing Academy - The private school library that "dumped its books"
You must consider how the school uses the library. Are students assigned opportunities for independent research? Are the classes mostly taught from textbooks? Do the students use the internet for research?
Some schools are close to local libraries. Concord Academy is across the street from the Concord Library. The school has a lovely library, but a student who needs to do research can also cross the street to access books at the Concord Library and all the other libraries in the Minuteman Library Network.