It's not about negative incidents, so much, as about not ever seeing anyone that looks like them, having role models etc.
So if they have a large family locally that looks like them, it's just by chance they are in this class alone and the other classes are more balanced, and the area is balanced, BME teachers - should be fine.
If this is the only child of a White parent and a BME parent and the BME parent is not on the scene and neither are those grandparents/cousins etc. and the area is all White - very worrying.
(note: Caucasian is not seen as a particularly helpful term these days, just say White)