My understanding of "sempai" was that it was usually one-half of the "sempai-kohai" relationship - where the sempai is a senior who took a kohai (junior) under his wing. I don't know if the same relationship traditionally exists for women - it may, and the terms may even be the same, or different - I only observed it in males in Tokyo.
In my experience at Hiroshima University, the sempai-kohai relationship transcended sexual differences. There were two broad types of student clubs: the 体育会 (Tai-iku-kai or sports club groups) and the サークル連盟 (Circle groups). There were aikido clubs in both groups. The two clubs with which I was most closely associated (Tai-iku-kai Aikido-bu) and the ESS (English Speaking Society) were in the two different groupings, but the kohai addressed their seniors by the 'Sempai' title, even if the kohai was male and the sempai was female.
There were subtle differences. It was not a general policy in the Circle Renmei clubs for kohai to use keigo (especially polite Japanese) when addressing their sempai, but in the Taiikukai clubs the practice of using keigo was the rule and was fairly rigidly followed (the rigidity also varying from club to club).