I think yabu OP. It is not a crime, but a teacher should always consider the individual circumstances and sensitivities of young children and if they don't they should be villified and be made to consider their position as a teacher of young children.
I too would be absolutely fuming if my dd's teacher took it upon themselves to inform the whole class that FC isn't real from the question of one pupil.
Dd1 is 8 and does question whether FC is real but she still believes, 'cos she wants to. I would be very annoyed if another adult took that away from her.
She knows that all the Santas you see before xmas at grottos etc are not the real thing and are just men dressed up as FC. I've never tried to kid her on that one. I think it's reasonable to explain that these people are there to make the build up to xmas exciting and that very young children may believe that they are really Santa.
When she asks about whether he is real I tend to answer with questions of my own. Only this evening she mentioned something about him coming down the chimney and I asked how he might manage that - like is that really how it works? I also say to her that since it appears that no-one has never actually seen the real FC all the ideas of what he looks like, or is, could be way off. I don't expect her to believe the impossible and I am also not lying outright. It's almost like putting doubt in her mind, and I'm hoping she would wonder, being the intelligent girl that she is, why I would do that if I KNOW he isn't real. I want her to believe until she just cannot convince herself otherwise or until she finds out with solid evidence. It's the thing that puts the real excitement into xmas for children. Yes, xmas is still lovely once you know but the real, magical excitement has gone.
At the age of 8 I think that my dd (and most other kids) would be more likely to believe that FC brings her presents on xmas eve night than that we could, and would, conceivably afford and actually buy all these toys etc that they come down to on xmas morning. I like to feign slight annoyance that Santa has brought all this stuff when they already have enough and we don't have room for more. It all adds the to the image that I don't have anything to do with it.