In English, "senior" was initially used to describe someone older or someone in a position of authority. 

Application to Education:
By the early 17th century, the term began to be used in educational settings to describe an "advanced student". 

The word "junior" comes from the Latin word "iunior," which means "younger" 

Sophos (σοφός): Means "wise," "clever," or "skilled".

Moros (μωρός): Means "foolish," "dull," or "senseless".