Do you think every ingredient in Sudafed is illegal? What about other cold medicines? Think they can test for every single substance that ever enters your body? Do you think they know what she ate for supper the day before?
Now, on the other topic: Do students check with their teachers to see if working together is acceptable every time they do an assignment?
Answer: No, they don't, because although school makes up a large portion of their lives, their whole lives do not revolve around it. They make reasonable assumptions and live their lives.
I am good enough that I get drug tested several times a year. I can assure you that I had absolutely no idea that cold medicine was illegal and if I had a race while I had a bad cold and someone gave me some cold medicine I probably would have taken some. Why? Because I would have assumed that something that was taken in reasonable amounts from an over the counter medication to a common ailment would be legal. It is a reasonable assumption to make.