The calpis you made is too diluted, Sensei~! It's normally five-times as strong, you know? Calpis is a milk-based soft-drink. But it also colloquially used as slang for semen due to its slightly acidic taste. An alternate reading of this might be "Sensei's semen is too thin~. It's normally five times as thick, you know?". The joke here though would be that she's saying something that can be interpreted as lewd without realising it.