@Bluedudu yep, completely forgot that difference between American and japanese school system. Or at least the american schools I am familiar with, where the classroom is the teacher's domain, so all the books would be there. Unlike here, where the gradebook could easily be at their desk in the staff room.
@icarushector in countrys where we have things like data protection the book with the grades is usually locked away unless he needs to write down new ones^^
@starburst98 And for the sake of the story, I doubt the teacher cares too much. If he checks his records, there'll only be a few questions and some confusion before they shrug it off altogether as some sort of mistake that's easily rectified. That is, unless Yada decides to do this stunt repeatedly.