The best way to improve high school vocabulary is to read and write regularly, and take note of new words you encounter in the text. It may help to write down a list of new words and look them up after you finish reading; then, going back to read that same passage will help you understand the word in context. Read interesting books, articles, essays, and so on; this will help you engage more actively with the new words, and whenever possible, you should try to improve your high school vocabulary by using the new words in writing or conversation.
High school students often need to improve their high school vocabulary in preparation for important examinations. Whenever possible, take advantage of free vocabulary resources available through the school's English department to improve high school vocabulary by studying lists of words you are likely to encounter on those examinations. Your teachers should be able to furnish such lists for you, but if they cannot, many such lists are available on the Internet for no charge. You will probably have to look up some of the definitions yourself, so be prepared to do a bit of the leg work.
Repetition is an important way to improve high school vocabulary. Repeat the word and its definition several times over the course of a few days to ensure it stays with you, and whenever possible, use the word in conversation or writing. Using the word regularly will help ensure you not only know the definition, but also understand how to use the word properly in context. One idea is to form a study group with peers and have a discussion in which each member is required to use three to four new vocabulary words properly. Make a game out of it: the person who uses the most new vocabulary words properly in a logical conversation wins the game.
Flash cards can help you keep track of the high school vocabulary words you need to learn. These cards make great study aids, and they are compact enough that you can carry them with you. You can also make notes on the cards, such as ways to use the word properly, images you can remember to help you keep track of the definition, and so on. Give the flash cards to a friend and have them quiz you on the definitions; reverse the process by having the friend give you the definition, requiring you to come up with the word.