I'll give a really brief overview of US school system (the public one, a lot of private schools do their own thing, but this is the "standard"):
Preschool/Pre-K
Before Kindergarten, so 4 or younger more or less. Basically this is more like educational day care than school proper.
Kindergarten
Age 4-5 or so. Often half-day, but I've heard of some full-day.
Grade/Elementary School
Grades 1-5 or 1-6, so ages 6 to about 11 or 12.
Middle School/Junior High
Grades 6-8, 6-9, 7-8, or 7-9, depending on where you are.
Ages anywhere from 11-15, depending on which grades are in there.
High School
Grades 9-12 or 10-12. So ages 14 or 15 to 18. Most places though school is only mandatory until age 16, so some people drop out before finishing high school (then later when you realize you can't get a job without graduating high school you can take a test called the GED which gives you the equivalent of a high school diploma).
Post high school graduation it gets more complicated. There are Community Colleges, which tend to offer 2 year degrees. There are Junior Colleges which (I believe) are also usually 2 years, but people normally go to those because either their grades weren't good enough or they couldn't afford to go to a 4 year university right out of high school, so they go to a Junior College first and then transfer to a 4 year university, because it's often easier to get accepted when transferring, and it's also cheaper. There are 4-year universities (colloquially referred to as "college") where you can go to get a 4-year Bachelor's degree, and then they also often have post graduate programs (like Master's/Doctorate programs). 4-year universities are usually made up of "colleges"; so like Kansas State University had the College of Engineering, and the College of Architecture, and the College of Arts and Sciences, etc.