Actually the Attribute types are defined first in the schema. So if your objective is to only create attribute types ofr test purpose you can do so without creating a Class. The objectclass attribute is utilised when you create the objuects of a class ONLY.
Whereas when you create a class in the Schema you would need the attributes from the schema.
In essence you cannot create a class in the schema without creating the attributes for it (if the attributes are not part of the standard schema).
I hope this is what you are asking.