It was a piece of school lore that was told to them by the upperclassman and that he later passed on to the younger classmen. They use it as an initiation process for high school. You are a real member of the high school if you know this joke.  

“My brother went to high school at [M], and always told us there were plenty of pranks people would play on my sisters and I. When I got there I was told there was a secret McDonalds on the 5th floor. It was funny because it was really hard to find the 4th floor so the idea that there might be a secret 5th floor felt pretty realistic. I remember a senior walking into gym class with McDonalds and telling the freshman he didn’t even leave the building. A lot of kids were really excited. I eventually told it to the freshman at the club fair.”

While it’s not uncommon for seniors to play pranks on freshmen, I think this particular prank developed because of the school structure. Upon asking more questions I found that the over 10 stairwells in the building all lead to different levels and sometimes skipped floors. This made it difficult for students to find the right floor when they first came to the school. Therefore the idea that a secret staircase to the 5th floor was pretty believable. It was a fun initiation for the freshman and a good way to explore the school.