My brother is a Freshman in high school now, and prior to starting high school he had heard that the older kids would always bully the little freshmen and call them ‘scrubs’. He said that the term came from the fact that the older kids would grab freshmen, carry them into the bathrooms, and use them to clean the toilet bowls like a toilet bowl scrubber.
He learned the term from his fellow classmates. They were fearing what it would be like to go through the transition from middle school to high school, and some of the kids had even heard some awful rumors about high school from older siblings they had. My brother knew about this specific rumor since it was the one that was repeated and talked about the most among the middle school students. For him it emphasized the fears he had of starting high school and how badly he wanted to stay young.
Usually terms like these, relating to high school, are discussed among the younger grade levels since they fear the unknown and do not know yet what they should expect out of high school. Sometimes these phrases are even used as scare tactics if they are told by upperclassmen just to prank the younger children and make them feel like they should be afraid of high school.
I remember hearing this term back when I was in middle school. I do not remember being especially afraid or worried though, even after hearing this phrase. I was actually excited about starting high school, and was not going to let any silly little rumors scare me before I even got to high school. It was funny that once I had started high school too, all of the rumors like this one went away and were not even mentioned. It was definitely something that was more emphasized in the elementary and middle school grade levels.