My child is 18, still lives at home and is still in High School. My ex says he doesn't have to pay child support anymore because she is 18. I Understand that the law in Indiana says child support continues until she is 21 or emancipated. Correct?

This web site suggests that the age of emancipation for child support purposes in Indiana is 19, changed from 21 last year:

How long will you pay? Indiana child support rules change July 1 - WSBT-TV (http://articles.wsbt.com/2012-05-18/child-support_31769516)

After age 19 the court may award ongoing child support to help pay for educational expenses such as college.

The rules he is under is what is specifically name and listed in your child support order.
So get out your copy and see. If you need it changed from what is written in it and signed by judge, you will have to get it modified