Additionally,
"It's high time (that) we left " means, we are still in the room, but in our mind's eye, we see ourselves in the
past, as having already left, and that's why "left" is used. "left" refers to the past, which is where are thoughts are: We should have left before now; We shouldn't have stayed until now.
"It's high time (that) we leave " means, we see ourselves in the
present, getting ready to leave. In that context, 'leave' expresses an unactualized event, something that hasn't happened yet, whereas with ". . . that we left", we project the event has having already taken place, even though it hasn't.