For a year it's not been a problem:) My own personal view is that it's safe, it has a factory fitted RCD, and a double insulated charging strip, however, the health and safety person has said that no child is allowed to plug anything in, having only just realised that trolley is switched on during the day. (Despite the trolley being used daily for the past year:)
Is it actually a problem if the children unplug them when they are live? The trolleys are wired so that the mains part of the charger is out of the way of the children, the only bit they can get to is the low voltage lead, which for most laptops is around 19v, and really couldn't do much damage!
A lot of our laptop trolleys are static, and therefore stay plugged in, with children in year 1 being taught how to remove and replace their own laptop. The newer ones come with an RCD, which could easily be added to an older trolley - so if there was any fault with 1 of the ac adaptors it would trip.