In high school, attendance is taken in each class, so your question is a moot point. He would be marked absent in the periods he missed. If a lower grade, they usually take attendance in the morning, so he wouldn't be marked absent. Just send a note that he has to leave early that day.
Is perfect attendance still a thing? I've put four all the way through MCPS and I never heard of attendance being a concern (or awarded). Is that what you are working on?