In grade school my teacher explained it was because people thought tomatoes were poisonous, and it would poison the actors. And when did this practice begin?
There were a lot of historical inaccuracies I heard in grade school, like how brontosaurus was too big to ever walk on land and had to live in water. That's why it's nostrils are on the top of its head, so it could poke them out of the water to breathe.
Your teacher was, um, likely showing some good creativity when pressured with a lot of curious children! But it came about after tomatoes were widely accepted as a common food. Previous answer you may like.