@starburst98 well chapter 1 shows so much of them nude that it was 90% certain they were female. Generally when you see THAT MUCH and see nothing it means Vagina.
But to add onto that chapter 3 has then age themselves up to a student's age and they are clearly female. They had no particular reason to be a female student vs a male one as they don't seem to even know enough about students to know the uniform was gender specific.
so it's like 95% likely they are a girl. The remaining 5% is incase the author is just trolling the hell out of us and intentionally misleading us while keeping enough hidden to have plausible deniability later on.
Also keep in mind they specifically say "reincarnation" which means it's not a direct resurrection of their former body. So it's not strange for the gender to change. And given how used they seem to be to reincarnation they probably end up flipping gender during reincarnation anyway that they stopped caring.