>begins using simple numbered grades after kindergarten
>only uses terms like 'freshman' and 'junior' and 'senior' as titles for specific grades
>it's not necessary to use these terms to denote what grade one is in

How the fuck is that difficult to understand?

Freshmen: Literally a "Fresh Man". The term dates back to the 16th century and literally means a novice or newcomer. When something is "fresh" it is considered to be new, empty, and inexperienced. Like a "fresh" sheet of paper.

Sophomore: Literally a "Wise Fool". Dates back to the 17th century and denotes someone who is more learned than a beginner, but still is still lacking quite a bit of knowledge.

Junior: Dates back to the 13th century and particularly denotes someone who is the "younger of the two", as in, when compared to a 4th year, they are the younger one. They are literally the junior of the senior.

Senior: This one should be quite obvious. It started in the 14th century and denotes someone who is the senior to everyone else. They are advanced compared to the others as they hold a "senior" status.