70 cases out of what I’m thinking must be millions of pupils seems very good to me, if they are “just” cases and not serious or critical cases.
Also, just because a child is at school, it doesn’t mean the child caught it at school! If schools are enforcing social distancing, I reckon it’s probable they caught it elsewhere when not socially distancing.
Arguably, opening schools could actually reduce the potential for transmission, ensuring that children mix in a socially distanced and managed environment, reducing the opportunity for some children (of parents who are not especially concerned about Covid) to mix closely with others.