Hidden Wifi How to Find One?

I want to know how to find a "hidden" wifi. By hidden i mean this: Yesterday when i was at my school where there also are some school laptops. The laptops had a special wifi named "(school name) cert" however i can't find the network on my own pc which my school have another wifi for (for home pc's). How can it be I can't find a wifi which signal is there but it's name don't show up so I can't connect to it? I would like some explaination on how it is done too if someone knows.