I'm trying to recall a story from my childhood. I believe this was an episode of a TV show, but could possibly have been a short film. I saw this in English in the mid-1990's.
The story involved some modern children (I think two brothers) who arrive at a somewhat Victorian-looking boarding school or orphanage. At the institution, the children are all served a very bland-looking gray mush at lunch. It turns out that this food contains some sort of drug that makes the children susceptible to mind control. After the children have returned to their bedrooms for sleep, the school administrators play some musical notes over the public address system to summon the children down to a subterranean level where they spend several hours each night taking care of dragons or other fantasy creatures being raised, possibly as part of some plan to take over the world. The plot is discovered because one of the new boys decides that the gray food is disgusting and never has any, so he is unaffected by the mind control efforts. He notices the unusual behavior of the other children, pretends to be mind controlled himself while following them underground, and destroys the baby creatures being raised in a fairly quick showdown.
I was reminded very much of this story when I read H. P. Lovecraft's short story The Festival as an adult, in which a protagonist is led underground to meet eldritch abominations, but the details don't match - Lovecraft's story doesn't involve a mind-control drug, and the underground area is underneath a church rather than an educational institution.