Is this a tern or a seagull?

[Catherine Wheel]

I've been told that seagulls and terns essentially look the same but that seagulls are only found near bodies of water while terns can be found where there aren't any bodies of water. So I'm wondering whether the birds that I see at the elementary school where I work (which isn't near any bodies of water) are terns or seagulls. Does anyone here know?