Low-grade and Second class

Low-grade adjective - Of low quality.

Second class and low-grade are semantically related In some cases you can use "Second class" instead an adjective phrase "Low-grade".

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

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