Actually, there is a very slight reference in the 
anime too.  When Ruby Moon shows up at Eriol's 
house for the first time, Spinel comments that 
Ruby is wearing the girl's uniform.  Ruby states 
that 'she' prefers the girl's uniform because it's 
cuter and that it doesn't really matter anyway.

Though I will always argue that Yue~san is 
male...but I don't want to venture into more 
*Spoiler* territory here!

^_~