I just finished my 4 years of IHSA riding for Kentucky. All schools are going to be different about the try out process. For example, my school never had to do actual try outs, we had more "placement try outs" so the coach could see you ride and figure out which division to put you in after evaluating your riding/ rider evaluation forms. Last year and this year however they had to have actual try outs because too many people were interested in the team.
IHSA divisions are as follows:
Novice Fences (2'3'' - 2'9'')
Intermediate Fences (2'6'' - 3')
Open Fences (2'9'' - 3'3'')
Your school is categorized into regions (which you will compete within for the season), and zones (you will compete with the top school in each region at Zones before going to nationals, which the top 2 teams in each zone go to nationals) It is all equitation based. The host school provides the horses, tack, etc, and you randomly draw a horse to ride (in our region, we use a bingo ball system but other regions do it differently). You do not get a warm up or any practice. You get to get on and adjust your stirrups and that is it.
Every placing (1st - 6th) is awarded a point value (7 points for first, 5 points for second, 4 points for third, etc.). You ride for individual points and your coach will pick a point rider for ever division to have their points count towards the team for the day (this is how your school earns points). When you point out of your division (36 points, except the open, then it is 28 points because your points start over every year) you qualify for Regionals. A first or second at Regionals qualifies you for Zones, and a first or second at Zones gets you to Nationals.
There is so much to say about IHSA. Personally, I loved it and I think it has improved my riding drastically. Lots of big time international riders did IHSA in their college years, like Beezie Madden (Cachione Cup winner too). Here is the website: About Us