OK, you both got "senpai" wrong. For example, in Ouran High School Host Club (pathetic choice, I know XD), Haruhi, a freshman, calls Suou, a sophomore, "Tamaki-senpai". Thus, when someone calls someone "senpai", it usually means they're of a higher grade or something like that...
Wow... The anime addiction in 6th grade really paid off XD Sorry, Loppy-tan~