History class is probably not the best time to attempt conversation. Gym class however, that is different.
I think (during gym class) you should approach her & say, "Hey (her name), do you got a minute?" She will either have a minute or she won't. HaHa! If she does, then just ask her if she would like to take a walk. If she reciprocates, then start walking around the gym talking or go sit somewhere (alone) & talk.
I don't really know what your intentions are. If you're wanting a date or her number, then that would be the time to go for it. Girls like guys who take initiative & who have confidence!
