i think he means that he changed all the plagiarised bits; but it still had the teacher's comments and the same length and everything; so the teacher couldn't prove that his work had been plagiarised because the hard copy he had wasn't plagiarised.
which begs the question; why did he plagiarise in the first place?
edit; got some hilarious stories regarding cheating.
back in the younger years, mostly in latin random quizzes when noone gave a shit.
like the time where we pasted notes onto the back of the teacher's laptop, and the projector at the top, and at the back of chairs, cos he never leaves his seat. then tell him afterwards and tis a huge laughup.
then there was this year; where the teacher enrolled us into the wrong exam (twas a global latin exam, no bearing on hsc) so we boycotted it by not studying; and we devised a cunning plan to transfer answers in the exam hall, which was being supervised by someone else; basically it involved; to ask a question, u coughed, then each tap of the pencil on the desk meant +10 to the question number, then clicking the pen was +1 to question number.
to answer, you thump the desk discretly, then use the same number system as before.
unfortunately, we happened to discuss these ideas in front of our latin teacher as a semi-joke, and he transferred the info to the supervisor who said before the exam, i quote, "if you feel the need to tap your pens or cough during the exam, please do so now."
good times.