Sorry, tracking until age 18 isn't long enough. Schizophrenia will show up a bit later if it does. Also, *having* a cat in the house is one thing - how many kids are cleaning the litterbox? Because that's the danger zone.
For those who are unaware, Toxoplasma gondii has a life cycle that relies on cats and their prey - typically rats or mice. In cats, it reproduces in the digestive system and gets crapped out. In rodents who come into contact with the cat crap, it infects their brain and makes them less afraid of cats, which benefits the parasite because it wants to end up in a cat's digestive system again.
In humans, it definitely causes miscarriages. There have been studies suggesting a link to schizophrenia, but I don't believe that's the current consensus. Something like 50% of all humans have been exposed to it, so it would be scary if so. But it might also depend on other factors.
It's conceivable that a parasite that has evolved to control host behavior could have adverse psychological effects on human hosts, thus the research into it.