Translation:There are a lot of students in middle school.
"Nui nā haumāna ma ke kula waena" could mean "The students in the middle school are big." You could also use other words that would make it clear that you were talking about size, rather than number, in place of nui. Puʻipuʻi, pūkonakona, or kanunu might be appropriate words, depending on the meaning you are trying to express.
I actually meant big as in big. I could easily imagine looking at a group of middle school students, some of whom veered towards fat and others towards tall, but all of whom were bigger than I expected of middle school students. I can remember being on a middle school football team and saying to teammates, "My god, the team from X school is big."