Useful for what, exactly? They’re no good for general research that third graders are going to do, and a teacher shouldn’t teach students to use badly out-of-date references when there are much better resources available. Maybe you could do one ‘compare now with then’ project, but that’s a lot of shelf space to use for a one-off project.
We don’t keep old scientific texts around as classroom reference materials (unless the class is specifically about the history of science), we keep original texts in museums and convert their contents to electronic format to preserve the data.
If your shelf space is not at a premium and you think preserving various semi-old encyclopedias is important, you can fill all of the shelves you have for free or very low cost by looking for people trying to get rid of them. Sounds like you have a new hobby idea!