Second-rate and Second class

Second-rate adjective - Of average to below average quality.

Second class and second-rate are semantically related In some cases you can use "Second class" instead an adjective phrase "Second-rate".

Second class noun - Not the highest quality in a classification.

Second-rate and second class are semantically related in low topic. You can use "Second-rate" instead an adjective phrase "Second class", if it concerns topics such as inferior, poor.