'Til is slang (informal) for "until," so it actually isn't an official word. Both "til" and "until" mean up to.
A person might say: "It is 8:00 now. I am in school 'til 2:00." They are in school from 8-2.
Still is also used with time. It means even now. A person might say: "I am still in school." They are at school when they say this.
Until and til are the same. Until is the formal/correct word. A teacher might say, "You have until tomorrow morning to finish your homework."
Yet means up to now. A person would say: "I haven't turned in my homework yet."