I have a school assignment where we're working in photoshop and will be printing the images. They'll be graded on their quality so need to be 300 dpi and minimum width and height need to be a minimum of 15 inches. I've used some images from google images and what I'm wondering is if I take an image that is pixel dimesnions: 40.0M, document size: 24 x 18 inches, and resolution 180 ppi, if I unclick resample image and change the document sive to 20 x 15 inches, giving me resolution 216 ppi, when I go to print will this be good enough quality? I know it won't be 300 dpi. Is there a way to upsample/resize the image so that at least one side is 15 inches and give me 300 dpi quality? I'm still really confused about and trying to understand resizing, resampling, etc.
If it's not against your assignment guidlines go ahead and check 'resample' and set it to 300ppi. Then print it, or just a sample strip. Then print a sample of your 15x 'x' @ 216ppi one. I'll bet you a Mt Dew you can't see a difference between 216 and 300. Practically speaking (your mileage may vary) It's when you start with very low end res (72ppi) and try to upsample to a too big size/ppi like 40 x 'x' @300ppi that you will see scaling artifacts.
Rule of thumb - don't upsample, unless you have a class deadline