And who knows how many guys she's done since then, huh?

 According to Planned Parenthood - who claim to be experts on this knda shit, Pregnancy doesn’t start the day you have sex. It actually takes up to 2-3 weeks after sex to become officially pregnant. 

It can take up to 6 days after sex for the sperm and egg to join - if a sperm cell joins with an egg, it's called fertilization. The fertilized egg moves towards your uterus. It gets to your uterus about 3-4 days after fertilization, and it can float around in your uterus for another few days. 

Pregnancy officially begins during implantation - when the fertilized egg attaches to the lining of your uterus, and the hormones needed to support pregnancy are released. It can take 3-4 days for the fertilized egg to completely implant itself in the lining of the uterus. 

So, if she was screwing everybody in her dance class, a couple of DNA tests 

might be called for. Juss' sayin'...