You need a second heater to be safe. I lost a fish like this, the heater just couldn't keep up.
I also got saved another time buy having a second heater. I did a water change, so I always unplug everything, well one of the heaters didn't get plugged in good so it fell out the socket. I came home and my tanks was 72 when its normally 78 - 80, so I assumed the 2 heaters couldn't keep up, I went to throw in the third heater, and thats when I noticed the problem.
Moral of the story is two is always safer then one, not only will this help you keep your tank at the right temp, but if one fails it might keep the fish from dying. Can you imagin how cold the water would be if there where no heater in there? When the water is that cold the heater has to work over time, if its on for 6 or 8 hours straight while your at school it could die sooner.
Also you might show Granny that the price of that water heater running for the whole time she has the heat off, actually cost more then having the heat on in the room. Just my 2 cents.