In Illinois, does the primary custodial parent in a joint custody get to choose in which school the child will be studying?

Did you have a parenting agreement or an order from the judge? Read it to see if it addresses parenting decisions like the one you're dealing with. It's hard to say for sure without more facts, but in general joint custody means joint parenting. In other words, both parents work together to make major decisions such as which school their child will attend, regardless of where the child lives most of the time. Contact the attorney who represented you for the original custody decision. They should be able to answer your question more specifically.