Answer: Reasons :
being too hot
being very upset, angry or in severe pain
heart problems
not eating and drinking enough
i'd say the reason for fainting most commonly would be low blood pressure.
Especially if you haven't eaten anything in the morning and are very skinny.
In my middle school, a lot of girls would faint when we were standing outside IN THE HOT SUN for the assembly every morning because they haven't had breakfast and were skinny.
So what to do?
lay them down and loosen any restrictive clothing.
Check for their breathing(they need sufficient oxygen) and raise their feet about 30 cm.
AND DON'T LET THEM GET UP TOO QUICKLY after recovering cuz they might faint again