Today is a special celebration day -
penkkarit!
Picture from Yle.Penkkarit (or
penkinpainajaiset, 'pressing-the-chair') is the last school day of the 3rd graders in high school (or
lukio). The name refers to the reason why their school year ends so soon: they're given
lukuloma or a break for studying for the final and most important of their exams, the matriculation examination. The break is indeed needed because these exams will cover everything that has been taught during the three years the students have spent in high school. The grades will also determine who can continue their studies in university, which doesn't help with the stress at all. If you're interested, you can read more about the matriculation examination
here.
But before the nervous last-minute-studying, the students take everything they can out of their last day in school. The exact traditions vary school by school, but usually the day begins with some sort of tricks or mischievous plans carried out by the 3rd graders. For example, interrupting lessons and "redecorating" the school are more or less popular things to do.
My school had a tradition of letting the 3rd graders do the morning greeting, which is usually a short speech followed by music and broadcasted via the school tannoy system in each morning. Well, our music teacher had a CD full of folk / aboriginal music across the world - everything from Mongolian throat singing to American folk songs. We took the CD, put it in the player and went to the gym hall to prepare for the rest of the day. Later on, a friend of mine (who was in the 2nd grade back then) told me the teachers had not stopped the music as we had thought they would, but instead they let the CD play for the whole hour while they continued with their lessons like nothing was out of ordinary.
Another thing the 3rd graders like to do is to reward the teachers in some way. It can be a play or a humorous short video or parody songs that are sung to the school - again, traditions vary. In Southern Ostrobothnian high schools, the students most commonly sing well known songs with their own lyrics that describe the teachers and the staff. The 3rd graders dress in folk dresses for this, which is not done anywhere else in Finland as far as I know.
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Picture from Yle.If you can't find or don't want to wear a folk dress, you can also wear
jussipaita. Although it's not official folk clothing, it's become something like a symbol of Southern Ostrobothnians. You can see a few
jussipaitas in the picture above if you look closely.
Picture from Wikipedia.The best part of the day is the truck ride. The 3rd graders dress up in costumes*, get on trucks, and throw sweets to other students, pupils, and people who've gathered to watch as the trucks go around the city/town. The routes are usually planned so that the trucks will visit school yards, so that the kids won't miss it. (After the trucks are gone, the kids always count how many sweets they got. There's always some poor soul who didn't get any or only got a few. At least my teachers gave their sweets to those kids. I'm sure there's some way of balancing out the sweets distribution in other schools, too.)
The trucks are decorated with banners* that the students paint themselves. There are some popular jokes that circulate every year, often based on the word
abi (short for
abiturientti, a 3rd year student). One very common theme is joking about your own lack of skills, how little you've learnt, or how you plan to take the exams easy. Memes are a big source of inspiration, too. Some jokes refer to the school the students are from, often making puns with the school's name. Some refer to politics in satirical way. Sometimes the jokes can be a combination of all of the above.
* not every school has the costumes / banners. When I was on the truck, we met a truck from another school which had neither the costumes or the banners (the truck had its own ads, which they weren't allowed to cover). You can imagine how much we yelled and jeered at them. 
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Pictures from Yle.MAOL is basically a cheat booklet you can take with you to maths, physics, and chemistry exams. It's almost like a good-luck charm for many; at least nobody in my class was ready to leave it home even when our teacher claimed we wouldn't need it in our exam.
Abitti is a digital course exam system that is used for course exams and the matriculation exams.
YTL is the Matriculation Examination Board, which everyone "hates". They're the ones who will make and grade every matriculation exam taken by the students. More info
here.
After the ride, the students go to a cruise. The main point is to drink, party, and spend time together. The cruise goes either from Turku or Helsinki to Stockholm and back, or from Helsinki or Turku to Tallinn, where the students spend the night in a hotel and party in the city. I'm very sorry for the citizens of Tallinn...

The next day is the time for
vanhojen tanssit (also known as
wanhojen tanssit,
wanhain tanssit,
vanhain tanssit or simply
wanhat). Since the 3rd graders are gone, the 2nd graders are now the oldest in the school. To celebrate this, they perform ballroom dances to their parents and to the students and pupils of nearby schools. The dances chosen for the performance vary by the school, but I doubt you could have
wanhojen tanssit without Cicapo or Kehruuvalssi! (Warning: these are both earworms.) The students have also created their own dance, which is meant to be funny and entertaining. The dancing takes the entire day: because everyone wouldn't fit in the same room at once, students usually give 2-3 performances in total for different audiences. In some schools, the students and their teachers go to have drinks together after the dances; but quite often, the students will spend the rest of the evening in their own groups of friends.
Cicapo & Kehruuvalssihttps://youtu.be/GYUppMF7AF4Dance made by the studentshttps://youtu.be/ncdHSZ9glGI