Junior and Schoolboy

Junior noun - One who is of lower rank and typically under the authority of another.

Schoolboy and junior are semantically related In some cases you can use "Schoolboy" instead a noun "Junior".

Schoolboy noun - A boy attending school.

Junior and schoolboy are semantically related in young man topic. You can use "Junior" instead a noun "Schoolboy", if it concerns topics such as person under legal age of maturity.