Go upstairs to Sam's room and open her locker with combination 0501. Grab the basement key, head for the basement door, unlock it and go down. Few rooms later and the safe is down there against a wall. Combination is 1963.
Easiest way to not pick up or investigate anything else in case you accidentally see the code is to just start a new game and do this. Should only take a few minutes tops. In particular avoid investigating the height chart against the wall as the last year on that is the code.