actually, from the careers tab, its not the box that you need to click but the picture bubble of your teen. clicking the box surrounding the bubble picture of your teen takes you to your teen, you need to click their faces be able to enrol them.
But when I un-enrolled my sim and enrolled him again, I had to click the box~ =_= confused so just try doing it both ways the next time
TV: 11,307,969 HFN, cat paradise, pet central : In-progress Reindeer park: #3 East Side beside the Real State Company HH: #21 West side just beside the starter house