Combining MrTango and LEMon's replies:
"Boss, I would be happy to try to complete as many of the new files you have given me as I can before I go back to school, but since it has taken me all summer to do less than half that number, I won't be able to complete them all. Who would you like me to train to complete this second phase of the project?"
When you do your write up, call what you did 'Phase 1' to distinguish it from the additional work. If it was a paid position, I'd consider offering X hours/week during the school year to complete it but unpaid? Forget it.
POD to Outdoor Girl. I agree - look at it as a 2 Phase project - you have completed Phase 1 successfully in the timescale given, and you are willing to make a start on Phase 2 and train up somebody else to complete it. Alternatively, if you are willing
, work out just how long it would, realistically, take you to complete the work on the new files, and amend Outdoor Girl's wording slightly to say "Boss, it has taken me all summer to complete Phase 1 of the project, which comprised 60 files. I am happy to take on Phase 2 which involves 150 additional files, but I go back to school on X date, after which I will have significantly less time to work on this. This means that I will not be able to complete Phase 2 until X date [enter a realistic future date here - even if it is 6 or 9 months away]
, alternatively, I can train somebody else to take over the project, which would you prefer?"
That way you are not refusing to do the work, and you are providing a solution to the problem.