It's four years and there is no national curriculumn. Each state sets the minimum number/length/type of courses required to graduate but then schools within a district (esp private ones) might require more classes than that. For example, in the state of Ohio, where I grew up, there is no requirement for a foreign language, but I went to a private Catholic school that required two years of a foreign language. (Districts are usually but not always organized by county within a state.)

Here are thee min. reqs from Texas - http://ritter.tea.state.tx.us/student.assessment/resources/grad/2008-09_GradBro.pdf

A semester is half a year. A course can run for 2 semesters (the full school year) or just for a semester. It'll vary between districts, but in general, the first semester runs from end Aug/beg Sept through mid-January with a week or so off at Christmas. The second semester runs from mid-Jan to sometime in June with a week off for Spring break.

Homeroom is a waste of time. At my school, it was 10 minutes each morning, first thing, where they took attendance and made announcements. At my brother's school (the local public school), it was 15 minutes but wasn't first thing - it was after the 3rd period.

I'm happy to answer any questions - fleurlb at gmail.