DDoS school server?

I am attempting to figure out how to effectively DDoS my high school's server (the server hosting the internet connection to every computer in the school). I have LOIC and a few friends who will join me with LOIC, but I'm not so sure about the target.
The IP address of the server is- 10.88.0.13:1347
I understand the first 4 numbers, but the :1347 at the end is confusing me. What is that? do I need it to effectively target LOIC?
NOTE- I'm doing this as a senior prank for 1 day, Please don't tell me not to do this. If you don't have any useful info, don't answer my question!