помогите перевести с английского на русский правильно еще

сама я перевела, но не понимаю смысл. помогите пожалуйста точно перевести.
public school- an expens private school where students usually live as well as study.
a school that is control and paid for with goverment.

Elective subject- a course of study that you choose to do,rather than one that you must do.
Semester-one of the two periods of about 18 weeks that the school year is devided into in some countries, for example, the US.