Asking about passing or graduation rates is not a bad question to ask, this will be costing you 50K a year all in all and 4 years of your life, you better be dam sure they are preparing you.
First off ask what you really want to know, there has to be something you want to know (in all honesty the student ambassadors usually have more info that you want to know). If you do need some fluff questions here are some ideas, do they dual degree programs or research, what research are you doing, what is they schools 5 year plan, have there been any major changes in the previous 5 years, do you practice now, what research do you perform.
Honestly though ask the student ambassadors anything, they will give you the "real' lowdown or other gems of wisdom i.e. which books you need to buy and which one you really do not, where to live. Most of what the committee or faculty have to say can be looked up online.