A few questions about tsc

Well not exactly tsc but the while story. I CANNOT seem to understand what exactly does the senior/junior/fifth-year and everything else mean. Are the upperclassmen the same as fifth-years? Are seniors also fifth years?

While reading tsc there was a moment of Jeremy being surprised that Jean is a junior, and then later saying in the interview that he's starting his senior year with the Trojans. So how many years younger is he than he's supposed to be?? So Jeremy is about 22 but Jean is 19, are they "supposed" to be the same age or not? I'm just so confused about the ages and years someone help me I'm begging 😭