Why do few high school girls leave their own pregnancy test kits in the school restrooms?

They most likely feel more secure there. If its found, it can be hundreds of girls rather than at home - they would be the main suspect. Also they can have their friends in the same room whilst upholding privacy; and school time (lunch breaks etc) give them the time and opportunity. Last thing any scared teenager wants is a parent walking in while they are mid flow, peeing on a pregnancy testing kit (or the wrapper being found)

I know people who have been on both sides of this problem. they don't want to get caught with it at home. and at school it could be like a hundred other peoples/

If they're hoping to hide it from their parents doing it at home would leave evidence.