This is key...ideally they would tell you the images dimensions (in inches or pixels) AND the dpi. Then you can give them the most perfectly precise image file.
But in the real world, I think whatever you do probably be fine. If you are using a photo editor that allows you to specify dpi at output, but all means do so.
If I were publishing a yearbook, I would be concerned about images coming to me with too little information/resolution...such as some mom using her 2.3 megapixel camera from 2003. In the real world, having enough pixels hasn't been an issue for a long time...Nikon's D40 from 2008 with 6 megapixels had a native image size of 3,008 x 2,000. As you can see, this would support 300dpi images of up to 10" if the image ration of 3:2 was maintained. Consumer cameras these days usually have tons more megapixels than that (let's not get into noise though!
As long as you're starting off with too much data and going down to a smaller size image, you're generally fine for most purposes. It's when you start off with too little and start stretching things that images can start to degrade.
Photoshop and other tools have algorithms that can intelligently fill in the gaps created when stretching pixels. But you do need to know what you're looking for to get best results.