Which white powder first "boils off" and then catches fire at high temperatures?
The fact that it turned black when heating tells you that:


It was an organic substance and did not burn completely leaving some carbon behind, or
It was a transition metal that had a lower oxidation state and was oxidised during burning.


Let's remove the obvious ones:


Not a nitrate. These will release toxic brown gas, which is not something you give to high school students.
Not an alkali (Na, K, etc) or alkali earth (Ca, Mg, etc) element because they're not pink nor they turn black.
I doubt it's a sulfur compound of any kind. You should have detected a sulfur smell when burning.


Maybe this can help you a bit.