Go and talk to you manager and, depending on whether you want to stay longer or not either say something like
Hi [Manager], Just wondering if you were aware that my Internship is listed as ending on August 25th and I don't think that leaves enough time for me to finish X, Y and Z that you assigned. I'm happy to continue past that date as I should be able to fit it around school if you are?
or
Hi [Manager], Just wondering if you were aware that my Internship is listed as ending on August 25th and I don't think that leaves enough time for me to finish X, Y and Z that you assigned. As I won't be here after that date is there anyone you want me to hand X, Y and Z over to?