Contrary to what most people think, they don't tell the whole school community if they have a positive case. They tell close contacts and some schools even encourage them to keep it quiet to avoid panic. Where it's an isolated case and no close contacts (because of absence on the day when symptoms started, for example), they usually tell noone and the family of the positive case often choose to keep quiet rather than be stigmatised as having broken the rules and caught it (I know you don't have to break the rules to catch it but that's the stigma).
Unless you have a particularly small school, you may well not realise that it's been identified in your school. My child had to isolate as a close contact of 3 positive cases and another parent of a child in another class said to me (during the time he was isolating) that they were so impressed there had been no cases in the school - yet 11 kids in our class had since tested positive!
On top of that, children don't usually show the main three symptoms. As soon as I hear about people saying how the only case in their school has been a teacher, I immediately know that it's probably spreading in their school without them being aware. Families talk about how they have no idea where they caught it as they don't go anywhere, except their child is in school 'but he's been fine so it hasn't come from school'.