This is same situation happen to my buddy. He tried to reopen the file and they rejected it again by stating the same reason.
Any thing before graduation CIC is not counting as experience. I am pretty sure, if you apply for PGWP before graduation and start working on PGWP, CIC should consider that experience. Based on this you can challenge your application.
Here is the simple steps:
1. Find your local MP and get appointment.
2. Explain them clearly and make sure you have perfect data from your side including dates. (You have to make them believe that VO has done something wrong).
3. That's it, Either they will communicate with CIC or They will give you letter on MP letter head stating your issue and request CIC to re-open your file. (remember CIC still can reject your file based on same grounds)
4. If you are so sure about your dates then go for it or simple re-apply you may get it faster.
My friend case he re-applied and got it for second time