In most cases the graduating class visits some of their more significant teachers over the course of an appointed night, and sing under their windows together. Unofficially, it usually means a night of moderate debauchery, over the course of which people get increasingly drunk, and the songs get increasingly scrambled. Some teachers tend to invite the entire class into their homes after the serenade and treat them to pastries and drinks before they send them on their merry way. Other teachers try their best to keep their location a secret and their infants/pets/neighbors undisturbed in their sleep.
My high school class, being a cohort of legendary overachievers, decided that we were going to go big or go home: We serenaded every single teacher on our collective list, which (because Hungarian education) meant teachers for Math, Advanced Math, Biology, Chemistry, Physics, Geography, History, Advanced History, Art, Music, English, German, Spanish, French, Religion, PE, and possibly a few others. Since teachers don't tend to live in communes, that meant we had a two-night trip around the extended suburbs of the city, from dusk till dawn, in a caravan of three cars crammed full with a number of students I should not name (Hungarian teenagers seldom drive at all). I only vaguely remember all the details; I was the only person with a guitar, so the trip was way more exhausting for me than for the rest of the bunch.
My mother has been teaching high school for decades, so I have had experiences with the receiving end of the serenade tradition as well. For most of my childhood years we lived on the eighth floor of an apartment complex, which did not stop the teens from serenading; they usually showed up around midnight, and treated us, the seven floors below us, and the two other apartment complexes framing the yard to loud and painfully long renditions of all the traditional serenade songs. As a kid, those were my favorite nights in the spring, although sadly I was not allowed to drop things on them from the balcony.
Talking about songs: There is kind of a canon that exists around the country that less creatively inspired classes can place their trust in. One of the all-time favorites is this gem of a song from the '90s:
The refrain very appropriately translates as: "This is who we were, wild and good, innocent among sinners; this is who we were, and there will be a sign that we leave behind when we leave..."
(I'd like to note that it is an unspoken rule that students shall not leave empty beer cans in the teacher's front yard)
More creative classes can present truly inspired performances: If they are lucky enough to have musically educated people in the class, they usually bring out all the violins, guitars and fanfares, and pick songs that are better in quality and in presentations. Also, classes get bonus points in my book every time they sing "Another brick in the wall."