You can get fired if they figure out you have been fudging the figures and 'lying' to them. I'm not sure how they would know, but if they do, it can create problems for you.
Error-level analysis of the Jpeg. As you say, if it comes out later that could have bad legal consequences - the term is 'detrimental reliance' and the dishonest person could conceivably face fraud charges. Even if not, it could easily become a a career-damaging topic of gossip.
Re: error analysis—it's a "picture of line-art" kind of document, isn't it? Not even a watermark. So: scan it at super high resolution, blow out the contrast, countour-vectorize it to SVG (effectively equivalent to recovering the original PDF), edit that, print the result, and then scan the print.
(It just occurred to me that if there was "an app for that" in this case, no parent would ever be able to trust their child's report card again.)
Don't even both with all of that.. Just make a reproduction from scratch.
Make it look more or less like the original. As long as nobody sees the original next to the mock-up, who's ever going to notice that one or two digits changed?
