The Witch University is the college for all the children in Halloweentown seen in Return to Halloweentown. In truth, it was the ancestral home of the Cromwells, but was later converted.
After Agatha Cromwell became an ordinary witch, the castle became an institute for young witches and warlocks.
Recently, Witch University enrollment was severely down, so Halloweentown Council and the School decided that for the first time in it's history they would admit non-witch/warlocks. The low enrollment of witches and warlocks has mainly Marnie Piper to blame, as she opened the portal between both worlds.