I keep the stl and gcode files around for the duration of the printing process. After that, I discard them because I'll have a bunch more files coming in next year. I'll keep the Tinkercad files around until the end of the school year just in case I need a reprint, but considering the students don't have 3D printing access outside the school (save for ordering prints from an outside source), I don't see any reason to clutter my student accounts with old files. That being said, I've kept the files for the more impressive projects in case I want reprints for examples or something of that nature.