Sit back and relax – there aren’t any. However, if you feel like improving your mind, the FAA have just produced a Special Airworthiness Information Bulletin regarding Fuel Contamination. I’m sure Google would find it for you or use http://rge.FAA.gov.
As most of the School aircraft live outdoors, be particularly cautious after heavy rain (or pressure washing) particularly if the fuel cap is the flat fit type rather than the ones with an overlapping rim. Have a look at the seals to be sure they are in good condition. If there is any water in the fuel sample repeat drainage after gently rocking the wings. If water persists after 3 samples then don’t fly the aeroplane!
Condensation is also more of a problem in the winter. In an ideal world fuel tanks should be left full overnight, but weight considerations preclude this. But after last flight of the day fill the PA28s to tabs and the AT-3s to 52 litres to minimise condensation problems. Some pilots like to take fuel samples after refuelling, but unless the contamination is gross, I think it’s better to allow 15 minutes for any water to settle at the bottom of the tank.