Right after graduating from his senior high school, Koichi joined the navy.
There was a senior high school, that was famous for celebrities going there, near the condo I used to live in when I was little.
I’m the vice principal of a senior high school.
Remember when you learned that 高等 means "high class" and 学校 means "school?" Well put them together and you have senior high school! It's the most high class school you've got (unless you go to university, but that's considered to be separate).
Fun fact: 高校 is the abbreviation of this word. Neat, right?
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.