Please can someone verify a 'fact' that my friend was supposedly told
at school, that baby chickens can drink through their bum (anus) - is
this true and how/why does it occur?

Many thanks in advance

I cannot prove it, but I believe someone is pulling your friend's
drumstick. My grandpa used to raise chickens, and I spent a lot of
time with baby chicks. All their bum activity was output, not intake.
Chicks sometimes like to sit in water and flap around, but they are
bathing, not drinking. Chickens have an organ called a cloaca which is
involved in both excretory and reproductive functions. The only time I
know of when a chicken's cloaca would be taking something in rather
than putting something out would be during mating, when the hen's
cloaca inverts and sucks in the rooster's sperm.

Some reptiles and aquatic creatures do take in water through their
bungholes. Perhaps someone was thinking of "Chicken of the Sea." ;-)

I am leaving your question open, in case someone else can find
evidence that baby chickens can drink through their bums.