I used Google to find the rules and the start of the season:
https://www.ihsa.org/Sports-Activities/Girls-Gymnastics
The school will certainly advertise when the season is starting, practices, tryouts, etc, but it may just be an announcement or flyer at the school. The skill level accepted often varies school to school, as some schools may have more higher level athletes and others may take almost anyone and then just not have 10.0 start values. Our xcel diamonds tend to do well in high school gymnastics, but the golds make the team too. I'm assuming you're posting on behalf of your daughter, so I would tell her to keep an eye out for anyone wearing a My High School Gymnastics shirt and then she could ask them if she wants to know ahead of time. Her club coach likely also has at least a vague idea.