An underclassman is someone who is in their first or second year of their undergraduate degree program. When they are seniors and in their final year then they are called upperclassmen.

US: Most US high schools and colleges are 4 years. "Underclassmen" (and "underclasswomen") are people who are in their first two years. "Upperclassmen" (and "upperclasswomen") are people who are in their final two years.