I'm not sure when the term language arts was first put to use. Language arts is a broad term and includes all fields of study that have to do with communicating with your native language. Language arts includes subjects such as:
- learning to read
- writing (composition)
- reading comprehension
The subjects that a child 'takes' varies depending on his grade level, for instances, a kindergartener will only learn to read and learn to write letters, most do not have spelling or grammar. Because of this, homeschoolers list the subjects separately when planning, but when they talk, they might just say language arts instead of listing everything verbally.
When you homeschool your high school child, write English I, English II, English III and English IV on the course of study and grade forms.