I know this is a bit of a bump, but I'd like to share my thoughts.

First and foremost that ADD/ADHD has nothing to do with how intelligent you are. I have an IQ of 140, so to give you some perspective 100 is average and 160+ is genius. (Granted I'm not sure how much stock I would put in the IQ system, but it's really the only "consistent" test.)

I was diagnosed with ADD(Inattentive) when I was about 12-13ish. I've been on and off medication since then. Shortly after I was also diagnosed with General Anxiety Disorder along with Clinical Depression. (The main thing to note is that the GAD & CD fluxuates, while the ADD is constant.) I am now 23.
The majority of my school career I was a C & D student. It's not that I didn't have a problem understanding, it was that it was my issue with focus/concentration, daydreaming/zone-outs, and thought processing & memory retention. Not to mention motivation or sense of priority.
So to make a Metaphor, it's like having a nice computer... but 50-60% of the CPU is being used for no apparent reason. Causing it to be laggy and have issues.
So as counter-intuitive as it sounds, school tests your knowledge and ability to learn. Not actually how smart/intelligent you are.

Here's something else to mention. ADD/ADHD actually has two different ends of a spectrum when it comes to focusing. Obviously the most commonly perceived end of the spectrum is the lack of focus. The other end of the spectrum is known as hyper-focus. Which means you are focusing on something that highly grabs your attention and interest, while blocking everything else out.
Here's what I mean:
ADD/ADHD Lack of Focus - The wiring in your brain is either slowed, impaired, or forced to take longer/alternate pathways because it's processing is either overloaded or dysfunctional. So the brain tries to remedy this in different ways. What I call the "Lock-Out State" is basically just zoning or toning out everything so it reduces or stops new incoming visual/auditory information. If you try to force concentration, it feels like it's really straining yourself.
Another is Daydreaming. Basically you try to think about something that you find interesting or makes you feel better in order to increase dopamine. Finally is the "hyperactive" or inability to sit still method. Dopamine is required for the movement of muscles, so the constant movement elevates levels slightly.
ADD/ADHD Hyper-Focus - This is what happens when you focus on something you enjoy or have a deep interest in. You don't pay attention to much else to anything else because said focus gives you pleasure/dopamine boost. TV or Video games are very big ones for example, because both are extremely stimulating and typically require little thought process.
Even today I am a big gamer because it helps me decompress. However because these types of things are so stimulating, they tend to be double-edged
swords because it interferes with your priority and motivational sense. Me personally I had trouble doing homework because TV or Video games was all I could think about.
People tend to confuse it with addiction. Here's the difference, addicts do it for the high. For example if a person that doesn't have ADD/ADHD but takes something like Adderall for example is at serious risk of abuse because of the high they get. People that have ADD/ADHD that take stimulants get the opposite effect(90% of the time), it calms them down.

Point being: People that have it do what they do or lack there of because it allows them to function. Not because they don't care or a lack of willpower. (Don't get me wrong, my long winded rant isn't about making this an excuse or a crutch. It's hopefully to provide a level of understanding.)

Despite being on medication, it's still an everyday battle. Personally it really sucks at times since I have to deal with 3 mental disorders not just one LOL