In a typical public high school, there are four grade levels deemed high school level, grades 9-12.

9th Grade - Freshmen
10th Grade - Sophomores
11th Grade - Juniors
12th Grade - Seniors

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For the most part, grades 9 and 10 are just like the previous grades with the exception of students starting to be able to drive (and thus accommodate that with (usually purchasable) parking spaces). They also try to push the whole 'responsibility of being an adult', landing it in the back of students minds as there really isn't much actual responsibility handed down to them at this time. 

In the larger, more populated schools, you could easily find a break here and have a whole additional school for the last two grades.

11th grade (junior) starts to set itself apart as the minority of students could easily be considered an adult. In the stricter or more troubled cases, it isn't unheard of to see kids with jobs or graduating early due to nabbing all the required classes early, or in the more extreme cases kids actually moving out on their own. (ya know: 16 and pregnant, parents are too insane/demanding, money's tight, ...)

12th (senior) is more or less for the lazy, the plain, and the underachiever. they only [i]need[/i] about 60% of their classes to graduate and they know that this is their last year within the system. Teachers usually are more relaxed with them, and they get by knowing that whatever they do isn't going to affect them, save for their academic achievements, ever again. Some don't really get too out of hand, but it is generally a chill year. Again, about half work and it isn't uncommon for there be an older love interest from outside the school.

suppose it would be a bit different for the rich; more hoity-toity and probably involved with the family business.
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not much difference, just how long they've been in school. 
as a private school, there could be most any kind of tweaks to the system in this rp