Yeah, in case you didn't notice, most teachers still keep regular old-fashioned pencil-and-paper grade books.
So if your grade is changed in the system and it gets noticed (entirely possible, and not unlikely except in very big schools) they're not going to have any trouble figuring out what the original grade was, or who the most likely culprit is.
If a bunch of students' grades change, the chance of detection is probably going to be an order of magnitude greater.
In the end, it's really not worth it to tamper with grades. Academic fraud is a serious matter. You really stand to lose a lot, and the chances of getting caught can be high. If you do get caught, you'll likely be expelled, and an expulsion for something like that is probably going to follow you around for awhile, especially if you're at the college level.
Besides, you're cheating yourself out of an education as well. You'd really be much better off just paying attention in school and doing your homework.
And eat your vegetables, say no to drugs and always go to church every Sunday!!