How do you get rid of pinworms without telling your parents?  

Why hide it. This is a common issue and not a sign of bad lifestyle or bad choices. There are over the counter pinworm treatments that work but are a bit messy & not always reliable. I commonly call in the simple (1)chewable tablet treatment if parents think their kid has them.
Tell your parents. Pinworms are not an std, but are caused by simply scratching the area around the anus or vagina. It is embarrassing, but you can tell them you want to see your physician and go - they are very common and are often the result of an outbreak at school, camp, or work where one person infects a great many through poor hygiene.