A lot of it is confusing and doesn't seem to make sense ... I do think there are some good reasons behind some of the rules but I won't digress here. I'll probably get corrected on this as there's always debates and definitely a lack of clarity from OHSAA, but my understanding could be summarized as this ...
* From the start of high school practice (late February) till the conclusion of a respective team's high school season, athletes can't play or practice with other teams ... but they can the rest of the year
* In the months of June and July, high school coaches are allowed 10 total interactions with their players, whether that's one at a time or all of them together
* In the fall and winter, high school coaches can interact with up to 4 players at a time, and as many as 4 players from the same school can play together on the same team
* In the months of November through January, high school teams can play indoors together as long as high school coaches don't coach them
Per the original post, a parent could be hosting practices of high school players if there is no involvement from the coaches most any time of the year except during the high school season starting in late February. I agree with Scooter that if a parent is doing this during the high school season, then by default they will be considered a high school coach, whether paid or not, and will be subject to all of the rules and regulations. And, oh yeah, they better make sure the high school coach is OK with that.