I'm not sure I agree with this. I think my own personal experience with two different abusive relationships has significantly clouded my views, though. In my first unhealthy relationship, the main element was isolation. Even when I discovered for myself that things were not okay, I didn't have anyone to talk to. I fell into severe depression, and I repressed every instance of sexual coercion until I could even begin to think about it a year later. I desperately wanted someone to notice and to talk to me, to reach out to me first and confirm my growing suspicions and dread, but I was so isolated that I figured no one approached me because no one knew. However, in the second abusive relationship I was in, I know people knew. The walls in my fraternity house are very thin, and there was enough screaming and crying and door slamming that there was no way the people around me could not know. But still, no one ever approached me, no one ever asked me if I was okay. There were people I had superficial "Hi, how are you?"/"Great, thanks!" relationships with who had to have known but never ever showed signs that they noticed or cared. That, I think, was what kept me in that painful relationship for so long-- the conviction that I had no one to turn to even if I left.
The rampant, wrenching self-doubt that pervades a victim of relationship violence is most enabled by other people's indifference. In both of my situations, I needed someone to approach me and say that they noticed, they cared, and most importantly, that the situation I was in was unhealthy, abusive, and not okay. It's not because I didn't know that I was in a bad situation; I'm a smart person who has read the literature and gotten the same talk as you did in middle school health class. Many people who end up in abusive situations are smart people who know, deep-down, that something is wrong. It might actually be their intelligence that binds them, because it suppresses their gut instinct, that little voice that tells them something is wrong, and causes them to rely on external validation. (Think scientific method, think burden of proof, think of all those things you learn in school. Education taught you to doubt what you think instinctively or what you're told until you receive absolute proof.)
Reading those lists of warning signs is usually not enough. Receiving general information about relationship abuse doesn't prepare you to diagnose yourself or find the strength to help yourself. Making that jump from a theoretical abusive situation to the one right in front of your eyes is one of the hardest things for an emotionally exhausted person to do. Even in what seems like the most clear-cut case to me now-- like the night when I was raped-- I needed someone to tell me it was rape for me to even try to begin feeling okay with believing it. I wanted so desperately to be able to call it rape, to put a name on it and to begin to distance myself from something I could identify as not my fault, but I needed someone else's input. I needed someone to agree with what the little voice in my mind was telling me. I just needed to hear it from someone, to have that particular combination of "I care enough to approach you" and "I notice too; you're not just making it all up."
Is that weird? Is wanting someone to come in and echo your thoughts, to be your mirror and your support, the exception instead of the norm? As I listened to these other students discuss ways to beat around the bush with a friend who is involved in something clearly unhealthy, I couldn't help but wonder why no one advocated being straightforward and calling it what it was. It definitely depends on the person-- I can see how some people might react with defensiveness-- but there are people who need to hear it from someone else to have the strength to take action for themselves. Please remember that these people do exist.
I wish I could describe how to tell if someone is like that. It breaks my heart to think there might be someone out there who is stuck and feeling helpless and wondering if people really can't see. I guess the best I can do is to say that if you're trying to decide whether to approach someone and ask if they're okay, do it. Maybe you're afraid that your friend might lash out defensively and stop talking to you; that's a legitimate concern, and sure, some people might do that, but please don't let that stop you from trying, because there are people out there who hope someone will take that first step and talk to them. It can be intimidating to try to talk to someone, and you might feel like you don't know the right thing to say, but as long as you're supportive and nonjudgmental, it's always helpful to know that someone notices and cares.