That is the grey area ... as of now, I don't know of any cases where that portion of the law was used or tested so without cases to provide interpretation you take the risk of being the "test case" which means you get the privilege of going through the court system and paying the lawyers to try to prove your point.
Basically since the zoo is covered by other "laws" I doubt they would try to use that one unless they really want to screw with you personally, but it is something to be aware of. More worrying for us here in Indiana is if we get tripped up by local LEO who try to use that if we are at a local McDonalds when a school field trip comes by, or other places that would not be thought of as a school function, but qualify on that day.
More of a political worry then anything else. Most of the local communities with the exception of some of the metropolitan areas are fairly civilian carry friendly. In fact the Boone Co sheriffs office is tied into the local "Gunsite Academy" (affiliated with Gunsite in Arizona) in their off duty time and provide some really top notch training for the public.
There is a book that I recommend for those who live or plan to CC in Indiana. "Indiana Handgun Law 2nd Edition" by Bryan Lee Ciyou, Esq. ISBN# 978-0-9786276-9-0. It not only walks you through the specific laws for Indiana it gives you a baclground on how to interpret the legalese that has helped me puzzle out the meanings of the laws in other states when I will be traveling.