In the USA, where undergraduate degree courses and high school commonly last four years, the following terms are generally used. They originated in England and fell into disuse there but are sometimes now adopted in other countries:
A freshman (common replacements: fish, fresher, frosh, newbie, freshie, snotter, fresh-meat, etc.) is a first-year student in college or university, or, chiefly in the United States, in high school. This word came from England but is now used far more frequently in U.S. English. The gender-neutral variation freshperson is rare.
At universities in the United Kingdom the term fresher is used to describe new students, however some universities are attempting to drop the connotative associations of 'freshers week' by renaming it to 'welcome week'. Unlike the American term freshman it sometimes only applies in the first few months or weeks of a student's first year. For the rest of the year they are called first years; the North American equivalent would be frosh (in singular and plural). The week before the start of a new year is called "Freshers' Week" at many universities, with a programme of special events to welcome new students.
The ancient Scottish University of St Andrews uses the terms bejant for a first year (from the French bec-jaune "yellow beak", fledgling). Second years are called semi-bejants, third years are known as tertians and fourth years, or others in their final year of study, are called magistrands.
Australian university students favour the term jaffy for freshman, literally an acronym standing for "just another fucking first-year." The meaning refers to the fact that, having finished high-school as one of the most important students in a small school, the new students are suddenly the least important students in a large university, i.e. they are "just another" in a sea of irrelevant freshmen.
It should also be noted that freshmen are generally picked on more than other grade levels, in particular by the seniors, because the freshmen are usually younger than the other students and lack general knowledge of the school. In many traditions there is a remainder of the ancient (boarding, pre-commuting) tradition of fagging. He may also be subjected to a period of hazing or ragging as a pledge(r) or rookie, especially if joining a fraternity/sorority or certain other clubs, mainly athletic teams. For example, many U.S. high schools have initiation methods for freshmen, including, but not limited to, Freshman Duct-taped Throw, Freshman races, Freshman Orientation, Freshman Freshening (referring to poor hygiene among freshmen), and the Freshman Spread.
Students are often stereotypically associated with childish pranks and japes.Even after that, specific rules may apply depending on the school's traditions (e.g. wearing a distinctive beanie), non-observance of which can be punished, even by a paddle line.
In the U.S., a sophomore is a second-year student, or, chiefly in the United States, in high school. Folk-etymologically, the word is said to mean "wise fool"; consequently sophomoric means "pretentious, bombastic, inflated in style or manner; immature, crude, superficial" (according to the Oxford English Dictionary). While it appears to be formed from Greek sophos, meaning "wise", and moros meaning "foolish", it is in truth from the word sophumer, an obsolete variant of sophism . In Britain, the term sophomore is unknown and second year students are simply called second years.
A student in deep thought.The term underclassman is sometimes used to refer collectively to freshmen and sophomores.
Similarly, The term upperclassman is sometimes used to refer collectively to Juniors and seniors sometimes even sophomores.
The term middler is used to describe a third-year student of a school (generally college) which offers five years of study. In this situation, the fourth and fifth years would be referred to as "junior" and "senior" years, respectively.
A junior is a student in the penultimate (usually third) year in college or university, or, chiefly in the United States, in high school.
A senior is a student in the last (usually fourth) year in college or university, or, chiefly in the United States, in high school. A college student who takes more than the normal number of years to graduate is sometimes referred to as a super senior. 
A student that is repeating a grade level of schooling due to poor grades is sometimes referred to as having been "held back."
The term pupil (originally a Latin term for a minor as the ward of an adult guardian etc.) is used in English primary and secondary schools instead of student, but once attending higher education such as sixth-form college etc, the term student is standard.
The United States military academies use only numerical terms. In order from first year to fourth year, students in these institutions are officially referred to as fourth-class, third-class, second-class, and first-class cadets or midshipmen. Some universities also use numerical terms to identify classes; students enter as "first-years" and graduate as "fourth-years" (or, in some cases, "fifth-years", "sixth-years", etc).
Freshers' Flu refers to the generic illness that many new students get during the first few weeks of starting the first year. This is often attributed to viral/bacterial diseases being carried by students from other regions of the world, to which some have no immunity.