Have you tried putting near some heated source to see how high the temp must be before the ink gets erased. dd and alot of her classmates used it during psle, no problem at all.
Yes, we did. It disappears even if you rub with your fingers or fades a little if you rub your warm hands over the text a few times.
From the Australian Pilot Pen website
Do not use the Frixion products when signing legal documents or cheques or for exams and other important writing.
Do not let the Frixion pens or any text written with Frixion ink be exposed to heat as this will cause the ink to turn invisible (this includes prolonged exposure to direct sunlight).
Pens exposed to heat can give the appearance of having run out of ink when in fact the ink is just not visible. If this does happen, place the pen or paper in the freezer for a short period of time for the ink to become visible again.
This person did an experiment with the pen:
She found out that the ink doesn't go away; only that it's invisible.
At 140 deg F (or 60 deg Celsius) the ink becomes colourless. This means that temperatures approaching 50 degrees will be faded.
The ink returns at -10 deg Celsius.
Like I said, ambient temperatures are something you can't guarantee once the paper leaves your hands. Temperatures in a car on a hot day can be temperature inside a parked car can be 30°C to 40°C degrees hotter than the outside temperature. Even if the papers are not transported or left in the car for long, they could be placed in a room near machinery like photocopiers, where the machines could get quite hot.
Even if the words don't disappear, if it's faded and illegible, the markers might not be able to see the answers.
It was fortunate that your daughter and her friends did not face any issues with their paper - but it's not a risk that is worth taking, especially after the kids have worked so hard.