Well you could always ask the nurse to "talk through" the flush, etc., the next time to help you "cement" what you are learning in school. It may be s/he doesn't know what's right, or just overlooked the flush. He/She may catch the error when they are teaching you....
I can tell you from experience, even if your NM has encouraged you, the bright motivated student, to come to him/her with info like this because you are their eyes and ears, you should smile, nod, agree and then keep your mouth shut. No matter how delicately you put it, no matter how careful you are, you are "low man on the totem pole," and it will come back to nip your backside!
If you feel you cannot tolerate watching poor care and errors, it may be time to look for a different unit or a different hospital, but I suspect that observing poor care and errors is a fact of life for nurses (which you already are, even as an extern).
I saw a lot of stupid errors, some of which constituted abuse and criminal acts. I kept most of it to myself. Maybe I should have been more open about it--hard to know in retrospect. I just made the best decision I could with what I had at the time.
For example, I saw a nurse grab a patient's arm and hold it hard against the siderail to get a blood sample the patient was loudly refusing (LOC was alert, oriented x3, etc., but she was a belligerant addict who didn't want to be there). Another cardiac patient repeatedly stated "I don't want you for my nurse, I am firing you" (something that particular nurse heard many times--and those were just the times I knew about!) and the nurse told the patient, "you have no choice until the next shift." How about giving IV push meds or drawing via a VAMP without swabbing the port? Going from bed to bed without washing hands? Leaving a patient to lie in their own excrement for the next shift to take care of?
We can strive to be better nurses ourselves, when we are no longer students. Until then, make your mental notes about how it "should" be done, befriend a nurse whose skills you respect and learn from him/her. Be glad you have the smarts and caring to want to recognize and provide quality!