While there was never any doubt that Kase-san is a lesbian from the get-go (she dated girls previously), I'm not too sure about Yamada.
Before and after she met her, it was never hinted that she would have been interested in anyone romantically or sexually.
So, I'd rather think Yamada started high-school as aromantic, then became Kase-sexual when she met her.
Something like love at first sight, instant crush, whatever.