My son was doing experiments at school with a D cell battery and touched each end with a paperclip. He said it shocked him hard enough to make his heart beat fast and knock him out of his chair. Is that possible?
As @jippie said, we're not doctors and cannot give medical advice. We also cannot give accurate advice without actually having seen this event, or even seeing first-hand evidence of what happened. Do not take this answer as the ultimate truth. The point being, don't blame me if I'm wrong. As is, there is not enough information in your question to do anything more than speculate. So, here is my speculation:
I seriously doubt that the story matches reality.
If each end of a battery were touched with a paper clip then there would be electricity flowing between the ends of the battery, through the clip. NOT through your son. Electricity just doesn't flow like that. But let's say that electricity was flowing through your son. How many times have you held a battery with your thumb on one end and your index finger on the other end? Did you feel anything at all? Nope.
The electrical resistance of your body, combined with the low voltage of the battery, prevents much electricity. There just wasn't enough to do anything. The paper clip cannot magically create more energy than what the battery already has in it.
But let's take this a step further. To affect the heart, the electricity would have to flow through the heart. As we established, the electricity was flowing through the paperclip only. If he was holding the battery in his hand then it would be flowing from the thumb to the index finger. In either case, not through his heart.
To be dangerous to the heart, the battery would need to have a much higher voltage and be flowing from his left arm to another arm or leg. I don't know what voltage would needed to cause harm, but I do know that most safety regulations consider things under 54 volts to be not so dangerous. Above 54 volts, devices need to have more safety features. Below 54 volts, there are not any more regulations than for a device running off of a 1.5v D-Cell battery.
Here is my prediction (made as a father of a 12 year old and a former mischievous boy):
He shorted out the battery with a paper clip and it got hot. He overreacted to the heat and fell out of the chair. The combination of surprise and falling, and an overactive imagination, made his heart get fast and his story get exaggerated.
Or, here is a different possibility based, unfortunately, on my own real-life actions when I was young: He actually shorted out something more dangerous, like the wall outlet. It made a spectacular pop, smoke, and light show, and possibly did experience some minor shock. He then made up a story to cover his guilt.
Again, this is NOT to be taken as medical advice! Get the real story from your son, but otherwise I doubt that he has any real medical issue.
Every electronics engineer that (nearly) zapped him/herself with mains power as a kid and denied to parents about having done anything that was forbidden raise hand (vote up). +1 for myself.
Don't do this at home!, but on the plus side, ever since that very day I have huge respect for these seemingly innocent copper wires. The shaking knees wore of in a couple of hours...
The story is improbable. Even if you touch two electrodes from a 1.5V source to your tongue, you can hardly feel anything. The voltage is too low to drive much of a current through the body.
Your son may have been pricked by the paper clip, but misinterpreted as a shock, because he was prepared to believe that it can shock him.
Being knocked from your chair can be the result of being startled, and a racing heart a psychological response. A mere abrupt sound can have these effects.