Yes that's the problem, that if only one person knows they will tell others and it's so easy to find out all the information online.It would take only 1 schoolchild to know them and it would spread throughout the school.
I don't think the average person on the streets would know who they are at all, none of my friends or family had heard of them other than what I told them. I was often in the same places as them when they would be out and about around Leeds but never saw them even after I knew about them. But most of their fans at their most popular stage were children and teens of a similar age to the girls, and are also now of an age to see all the things about Chris on youtube and tik tok and have the parents who found out about it through them too. So there isn't much chance that an entire school has never heard of them. Plus, being the super famous Inghams, they would soon be bragging about their youtube channel so people would soon look them up anyway.
From Isla's age down at least they would have chance to make friends before the others found out about everything or fully understood it all, which might help but they are all going to have to learn to deal with people talking about it at some point.