Yes, it is. Almost any school I know of deems it as cheating. It must be "original" work. AKA only used one time and one time only.
That being said, I have done it before, and didn't give a single shit. Just re-printed the saved file or copied off my own homework from before. I have a D-ring binder from every class I've ever taken along with the book. I do this for a lot of reasons, but won't go into that. You did the work, so I don't see anything morally wrong with it. Just like tuners re-using basemaps as a starting point, or re-working previous tunes they have done for a vehicle. It's only yourself you are hurting in the long run I guess. If you are confident in your knowledge of the material, then I see no issue.