Only time i've seen water in fuel is after a heavy downpour of rain or aircraft has stood outside for several weeks.
Always used to rock aircraft gently, then drain fuel long before student got his hands on it.
Put water in sample to show student what he might see, as it is rarely seen. Also the pour sample onto paper, avgas will evaporate, but jet fuel will leave a greasy spot.
Warn student not to get avagas onto hand, good way to get dermatitus.