I had the exact same issue and after trying like 5 times I finally succeeded. What I did? Well I had it saved from the day before graduation and followed this precautions: http://sims3.crinrict.com/en/2013/03/precautions-for-university-life.html
Basically this problem is caused by calling the sim's relatives and friends to attend the graduation. If you do so, the phone will be locked and you can't leave after graduation nor by dropping out. Also remember to empty the mailbox in case you have love letters there.
Here's more info: http://forum.thesims3.com/jforum/posts/list/655771.page
