My son's science teacher had 15 fifth grade students stand in a semicircle holding hands. The student at each end held an electrode connected to some kind of variable contoller. The teacher increased the voltage until they could "feel" the electricity. He also had one student in the middle let go to demonstrate a "complete circuit" Is this dangerous?
These sorts of experiments are safe so long as the teacher takes care to follow standard guidance such as Van der Graaf Generator Safety which goes into some detail.
The electrostatic energy stored by the sphere should not exceed 0.5 J.
An enquiry to CLEAPSS has revealed no recorded incident of direct injury caused by shocks from the correct use of school Van de Graaff generators.
and so on.
From memory, I believe a current value of 65mA passing through the human heart has the potential to kill. According to Hyperphysics, 1 mA is the Threshold of feeling, 10-20mA causes a "can't let go response" where the muscles contract, and 100-300 mA can cause Ventricular fibrillation, and can be fatal. The linked pdf stats that 6mA for a woman is enought to cause a painful shock. For the sake of safety, let's say we do not want to go beyond 3 mA. A typical resistance between two body parts is 1500Ohms Hyperphysics claims 100kOhms for the human body. Ohm's law dictates that if I held a potential difference of 300V across my hands, that the current could be dangerous.
The class was standing in series, so it would take 15*300V = 4500V to
kill pose risk to the students. I would hope the teacher wasn't using a source anywhere near that value, which is not exactly the most accessible either.
From Wikipedia, it is said that 5mA is enough to feel, as opposed to 1mA.
In short, I'm sure the experiment was safe, but extreme caution should always be exercised in regards to electricity. Also the above values that I explained should not be interpreted to find absolutely safe zones when dealing with current.
The plural of anecdote isn't data. Just because 100 of us answering the questions have done similar experiments doesn't mean that it's safe or recommended. I've personally received shocks in the hundreds of thousands of volts range (at very low current) and have been set on my ass from much lower voltage but higher current shocks and survived. It doesn't mean that what I survived was therefore safe or recommended. It means I was lucky, if anything. Now that I'm older and wiser I don't go out of my way to experience shocks, even if I'm sure they're safe. (The only thing I do still do is briefly tap a 9V battery to my tongue to see if it's got any juice left in it, but I've found that 9V batteries will "bubble out" on the bottom when they're dead and tend to trust that instead.)
In fact, I'd be very concerned about having students stand in a circle conducting enough current that they feel a (mild) shock. You never know if one of those students has an undiagnosed heart condition. Sometimes the damage caused by electric shock can take several hours to manifest, and it's doubtful that the people taking the victim to the hospital would connect the dots in such a case.
If the teacher wants to demonstrate the concept of a circuit he can do so with a moderately high frequency, low voltage AC signal and a sensitive detector which lights an LED. It'd be way safer than anything that you might actually feel and get the point across just as easily.