No, it wouldn't be plagarism unless you pretended you took the picture (or otherwise created it). Even then, I believe pagarism refers to a writing, not graphic art. You can avoid being accused of plagarism by simply putting a note in the project saying that you didn't create the screenshot.
Anyway, what is a bigger concern is copywright infringement. Use of the image in a school project may very well be permitted by the fair use excepton.
Please be aware that I may not be licensed to practice law in your jurisdiction, and therefore am not giving you "legal advice". You understand that an attorney/client relationship is not to be established.