Visitor Visa's are always difficult to predict.
I advise everyone the same, with the low cost it's just got to be worth the effort.
If you are the sponsor, then write a great supporting letter and submit evidence such as bank statements and pay slips etc. together with accommodation details.
Your g/f should also just supply all the good stuff she can.
I think your idea about submitting University documention and fees etc is also worth a try.
In many ways it seem like a lottery. You need to be able to convince the ECO and give them a comfortable feeling. Basically it's down to what opinions they form and how convinced they are. Solely discretionary.
But their number one priority is to be comfortable with a decision that says, the applicant will return.
Please do try.
Good luck
