My employer has asked for a written response regarding an arrest when I was 15 year old. I am required to provide one?

It is likely that the record is "Sealed" but not expunged. Juvenile records are usually "sealed" so that there is no way for anyone researching the record to know what criminal act you are accused of or convicted of, but they still can tell that a record exist.
The reason your employer is asking what is in the record is precisely because they have no way of know without you telling them. You have the right not to tell them. Of course that could mean consequences. There should not be any retribution for your refusal to discuss your juvenile record but of course what "should" happen and what does happen are rarely the same thing. It is likely that they may be using your juvenile record to justify an action to demote or remove you. For future reference you should ask the court where your juvenile charges were filed how to go about getting your record expunged if you want to remove it permanently.