“in” can refer to a location as well as to membership in something:

I am in fifth-grade class. (I am enrolled in the group of students which is designated fifth-grade class.)

I am in fifth-grade class. (I am physically located inside the room labeled fifth-grade class)

“at” can indicate enrollment at a particular institution (the physical classroom):

I am at fifth-grade class. (I am physically located somewhere close enough to the room labeled fifth-grade class, and I am not necessarily inside it)

Compare it with “study-at-university-or-in-university”