“Study at class 5C” or “study in class 5C”
I attended an elementary school in that similarly divided its students into classes designated by grade level and letter, with each combination also corresponding to a specific homeroom teacher and to a specific classroom. Whether you would use in or at would depend on whether you were referring to the grouping or to the physical location within the school building.
At generally refers to a specific location, whereas in can refer to a location as well as to membership in something. As such, in is the most likely scenario, although which meaning is intended would depend on context:
I am in 5C. (I am enrolled in the group of students which is designated 5C. In the U.S., one would more commonly use the homeroom teacher’s name: I am in Mrs. Smith’s fifth-grade class)
I am in 5C. (I am physically located inside the room labeled 5C)
While you would use at to indicate enrollment at a particular institution (I studied at the University of Alberta; the children are in elementary school at St. Ann’s), you would not normally use at with an individual section within that institution. You could use it if referring to the physical classroom as a point in space:
I am at 5C. (I am physically located somewhere close enough to the room labeled 5C that I would cite it as the nearest reference point, and I am not necessarily inside it)