You can write it senpai or sempai. Japanese people tend to pronounce it as sempai. My japanese girlfriend just said they say senpai, not sempai.

The terms senpai and sempai often spark confusion, especially for those new to japanese culture. While both words circulate in online communities and discussions about anime, manga, and. In informal use, senpai (also styled as sempai) can refer to anyone whose attention you want to get—that could be someone you admire and want to be friends with or someone you're interested in.