There are a few ways to do this. If you plan to use the same logic in more than one place then it would be a good idea to create a table in your database with a ClassID and ClassName so you can just join to this table in your query to return the ClassName.
If for some reason you cannot do this then you can do it with expressions in SSRS directly.
Your expression would look something like this.
=SWITCH (
Fields!ClassID.Value = 100, "Class A",
Fields!ClassID.Value = 200, "Class B",
Fields!ClassID.Value = 300, "Class C",
True, "Unknown Class"
)
The final True
acts like an else, so if none of the previous expressions match then '"Unknown Class"' would be returned.