Low-class and Second-class

Low-class adjective - Occupying the lowest socioeconomic position in a society.

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

Second-class adjective - Of little or less value or merit.

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