I may have missed it, but one possibility doesn't yet seem to have been considered.
According to an article by Brian Dunning, the media in Zimbabwe were first alerted to the Ariel story by a phone call to the local radio station (ZBC Radio), which had actually been calling for 'UFO' stories in the week before.
This raises the possibility that the whole incident was the result of a deliberate prank or hoax. Local people (probably but not necessarily youngsters) may have dressed up in improvised 'alien' outfits to spook the school children, either just for fun, or in the hope of getting money from the media. Or some of the older kids at the school may have dressed up to spook the younger ones. This might explain why in some accounts the 'aliens' are about the same height as the older children. We may also note the suspicion of the tuck-shop lady that the kids were making it up.
But the latent conspiracy theorist in me wonders if the incident may even have been devised by the school itself, as a publicity stunt. This would help explain two otherwise suspicious features: the apparent eagerness of the school (or its head teacher) to co-operate with the media, and the convenient 'staff meeting' that coincided with the aliens landing.
Some previous posts (including mine) have assumed that publicity would be unwelcome to the school, on the grounds that schools generally, and private schools in particular, would want to shield their children from media intrusion. But is it possible that the school had reasons of its own for wanting publicity? It is conspicuous that most of the children at the school were white, even some 14 years after the establishment of black majority rule in Zimbabwe (formerly Rhodesia) in 1980. But the white population was dwindling. Many white settlers emigrated after 1980, and according to the Wikipedia article on 'White Zimbabweans', the white population declined from 220,000 in 1980 to only 70,000 in 2000. Faced with a decline in its traditional demographic base, Ariel school may have been keen to attract a wider ethnic mix. (From its website pictures, the present-day school intake appears to be almost entirely black.) In this context a little media exposure may have been welcome, following the showbiz adage that 'there is no such thing as bad publicity'. Of course, the school may not have foreseen just how much publicity they would attract, but once that phone call had been made there was no turning back.
This is admittedly a far-fetched suggestion, but still not as far-fetched as tiny telepathic aliens.