Internship report - any way to request discretion from reviewers for a to-be-published paper?
I think it would be fair to say something like 


  This report concerns work in progress and should not be freely distributed pending publication of the results 


I also think it would be wise to talk to somebody in a position with insights into how the reports are handled. You may not be the first to have this issue. I am sure such reports are not circulated anyway unless specifically stated they will be from the beginning. But it is better to be safe than sorry so do not hesitate to ask around to get the necessary information you need.