From what I recall taking that bus was about two stops to be at the dorm end.If you continue taking the bus up Winston will end up in front of stonestown where it meets 19th ave. you can get a quick bite at mc Donald's or something at Trader joes and take a short walk to class entering through the science wing.
The SFMUNI 18 line bus is supposed to come every 20-30 minutes depending on the time of day, which way the wind blows and how lucky you are. The last bus is around midnight. The campus is only about 1.5 miles away by bike or on foot and about 2 miles by car/bus. The walk is actually quite enjoyable through scenic Lake Merced. I believe it is only 3 stops until the campus. If you continue past the campus you will end up at Stonestown Galleria where you can catch the M train downtown.