(shrugs) that’s my big sister told me when I was a kid. She had a teacher who was Jewish and didn’t know it, and they didn’t tell her for this reason. It was based on the idea that it’s better for a person to sin unknowingly than knowingly.
There may be nothing to it. I’m not saying my sister as a teenager was necessarily a reliable source.(I’m also not guaranteeing that I understood or am remembering the details accurately). But it’s worth checking out.
Also, if this was a matter that involved her class, there is a good chance that this came from the fathers of her classmates who were Talmidei Chachamaim. So it is POSSIBLE that there was something to it.