An Instructor pilot entered a TFR around a stadium during a sporting event; but the TFR's existence was not documented in the preflight NOTAM briefing.

Narrative

While giving local flight instruction and an area familiarization flight; I inadvertently entered a stadium TFR while on a discrete code with Norcal. The fact that there was a game going on at the stadium was not something found in the weather briefing and it completely caught me off guard. I assumed there would be no game due to baseball season being over. I am still not sure how pilots are supposed to know the stadium game schedules other than working with the controller and being advised of such; or knowing the entire schedule for all stadiums where they fly (this; of course; is just not practical). I was advised that the stadium TFR was in effect; but by this time I was already within the lateral boundary. I immediately exited the TFR. As a prudent pilot; it is expected that the pre-flight planning prevents any unnecessary surprises and therefore preserve the safety of the flight. However; this seems to me a hole in the pre-flight briefing system. It is easy to imagine how such a surprise in the cockpit could lead to a further chain of events such as busting airspace or unsafe maneuvering at the last minute. Any surprises in the cockpit have the potential for progressing into a dangerous situation and at the very least; this ambiguity leads to an uncertain mentality and erodes confidence.