Leader, Prime Minister
The neck of the squad is the most important part of the squad, for if you lose the neck you lose the head (they're all one thing). This person is the head or leader of a group.
This is a very formal word to refer to someone in a top role, especially a Prime Minister (aka the head of the cabinet).
This is a jukugo word so it uses the on'yomi readings. You learned the on'yomi for 首 in the words 部首 and 首位, and you learned the on'yomi for 班 with the kanji. Go you!
Who do you think will be the leader of the first-year students?