From upthread, (Parthenope, I think)
'I visited with Jane' isn't quite the same as 'I visited Jane'.
'Visited with' involves a good old natter. You could 'visit with' someone you sit beside on a long train or plane trip.
Visiting Jane might not involve an in depth or lengthy conversation.
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Youse is Irish as well as Scottish, but you wouldn't hear it in boardrooms. Hence its use in east coast urban areas in the US, and also Chicago. Also Yiz.
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Taking a rain check
A rain check is an actual thing. You go to the supermarket and find an item advertised at a certain lower than normal price is sold out. So you go to the customer service desk and ask for a chit allowing you to buy that item at the sale price next time they have it in stock even if it's not on sale. You produce the chit at the checkout and the cashier is supposed to honour it.
The concept started out with outdoor events, I believe.
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Freshman, Sophomore, Junior, Senior.
I would've thought Junior was the youngest? Why do they have these names as well as Grade 1, 2 etc? Confusing.
The Freshman, Soph, Junior, Senior years are high school terminology, and then you start again for university years.
Grade 1, 2, etc are for denoting elementary school and middle school and also used in paperwork for high school. So you get grade 9 (freshman), 10 (soph), 11 (jr), and 12 (senior). The numerical form is easier to fit into forms than the names are.
(In Ireland your first year of secondary school is First Year, not 7th class/year/grade).
www.merriam-webster.com/words-at-play/learned-fools-freshman-sophomore-and-the-rest
Origin of Freshman, Sophomore, Junior and Senior.
Also "The student body"?? does that mean "the whole school"?
Yes.
And GPA - Grade Point Average - is that your average marks for a subject?
It's basically the average for every subject, averaged, calculated over the four years of high school.
'Basically'... gpacalculator.net/how-to-calculate-gpa/
Everything you ever wanted to know about calculating the GPA, and good luck.

...and how many school classes do they have during a day? 8 or 9? seems an awful lot.
I believe there are eight class periods in the average high school day. In the high school I am most familiar with everyone does daily PE, accounting for one period. Then there is lunch, another period. Then five or six classes. If five classes, a student would also have a study period, aka study hall, very handy for catching up on sleep or homework, etc. If six classes, a student would have no study hall or might have somehow got a PE waiver so that they could take a class instead of PE and also have a study hall. Just in the school I am familiar with, doing instrumental music involves losing a study hall.
Doing 7, 8 or 9 or more courses is the norm for Ireland (more courses for Junior Cert, 7-8 for Leaving Cert) and also I believe Scotland.
Most US students take 5-6 courses per semester. Most courses last all year and are just semesterised into course descriptions like French 1-2 or French 7-8, each number denoting a semester. Some students would start at 7-8 as freshmen and some would start at 1-2 or anywhere in between really. Some courses are one semester long.
Many US high schools offer students the chance to do any course at any level they can qualify for, and allow required or mandated courses like financial literacy to be taken in any year, so you can do an art foundations course that has students ranging in age from 14 to 18, or a pre calculus course with students who are in the same age range..
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Yes 'quarter of' = 15 minutes before the hour, and 'quarter after' = 15 minutes after the hour.