Apple’s next splashy product announcement won’t be from its new campus in Cupertino, Calif. It will be from a magnet high school on the north side of Chicago — a signal that Apple isn’t ready to let go of the education market it once dominated. Apple has not said what products will be introduced at its event, but the announcement comes at a crucial time as many schools are looking to renew or change their agreements with companies for classroom tech over the summer.