Tough situation to find yourself in... particularly when you aren't confident enough to be this upfront with your parents.
The risk is you do it in the evening... get a false negative... your period never comes and find out much later you are in fact pregnant.
I just figure this is one of those times you would rather do it right and do it once.
Before they figure this out just by how you are acting... yes we old farts have learned to pick up subtle cues by peoples behaviour, reactions... and changes in their behaviour that the people being watched aren't even aware of ( a lot of us anyway).
That's why I say they might be more aware of things than you think they are... and the longer you drag this out the more likely they are to figure it out.
WITHOUT actually catching you in the loo peeing on a tester. If you see the point I'm trying to make.
I doubt they go through your purse.. and even if it's there a few days, you will likely get a chance to dispose of it soon enough. They aren't very big... they are in fact quite small.
.