Re: Early-graduate Sim can't enroll in university?
Have you tried checking application status on a computer? Whenever I get tired of waiting for the acceptance letter, I just go through the computer. It's like sometimes it just gets stuck in the mail and takes forever, but on the computer there's generally a result available in just a day or two, and you can enroll from that result without waiting on the letter. One time my Sim's official acceptance letter was delivered just a day before her first term finished!
I don't have high school though, so there could also be a glitch specifically in the early graduates.