Event scripts get compiled as a sub within a single vbs file for the business object. You cannot declare a class within a sub or function. Now, that being said, you might be able to get this to work by reading the Sharing Functions and Subroutines section of this article.

https://technet.microsoft.com/en-us/library/ee176984.aspx#EEAA

i once played around with the idea of having functions or subroutines that i could call from the event script so i didn't have to repeat a bunch of code in the event script as you can't declare subs or functions in an event script. Your mileage may vary. 

Sweet article. If I get a chance to try it, I will post experience.