generally it goes:
17-18 yrs old - senior year (or 12th grade)
16-17 yrs old - junior year (or 11th grade)
15-16 yrs old - sophmore year (or 10th grade)
14-15 yrs old - freshman year (or 9th grade)
13-14 yrs old - 8th grade
12-13 yrs old - 7th grade
11-12 yrs old - 6th grade
10-11 yrs old - 5th grade
9-10 yrs old - 4th grade
8-9 yrs old - 3rd grade
7-8 yrs old - 2nd grade
6-7 yrs old - 1st grade
5-6 yrs old - kindergarden
4-5 yrs old - pre-k

of course some people do get left back and have to repeat a grade which will earn them the title of "super senior"

the years between pre-k to 6th grade are elementary school children
the years between 7-8 are referred to as junior high or middle school
the years between 9-12 are reffered to as high school

though there are some schools that are small enough to host grades pre-k through 12 all in one building. but generally they are separated by group as listed above.