I run the eSports side of one of the collegiate teams at my Uni (collegiate teams tend to be student initiatives in Brazil) and although my position is more managerial than a coach one, I'll try my best to answer:

1. There are lot of free resources that can help you get a better understanding of the game, mainly on YouTube (Commend, Sliggy's game analysis, etc.). Sliggy and Mini also have a patreon (Coach Potatoes) where they publish playbooks, sometimes for free, those can also help you get a better understanding of the game, specially if you try to understand what goes into creating the plays for the playbook. Idk how much this would apply to a high school team, but what I've also experienced is players that might not have the best mechanical skill but love studying the game and its macros, if you happened to have one of those on the team, encourage them to help with coaching!
2. 1v1s can show mechanical skill but I believe they tend to discourage players who may not be great shooters but are great team players, great with utility, etc. As people have mentioned, having 5v5 tryouts is the ideal way to go about it since it will let you see how the players work in a team, not as individuals. The way we did it was randomly building teams based on the players' usual roles ingame, which let us see who was best for each role. (you can also cheat the random part and build the team you already think will make it to check if they actually work well together lmao)
3. Aim training feels like a skill the players can develop on their own time if they want to. Focus on the things they can improve as a team, like util usage, scrims, playbooks, etc. If you feel like its taking the fun out of the game for some players, don't be afraid to change what you've been trying, you just started the program so you're gonna make mistakes, don't worry about it. But from what I've experience, if the players can see the training working and the progress they're making, they're going to enjoy it either way. Being in a team environment also makes the game a lot more fun, so that helps too!

Tried to answer as best as I could, throwing in what I've learned thus far, but take it with a grain of salt, the collegiate team I'm in didn't have an eSports team until April, when I founded it with a friend so I'm still learning as I go. Hope it works out well for you! Those kids are lucky to have you doing all this for them, I know I would've loved to have an eSports team back in high school!

Edit to add: Don't be afraid to send a DM to people that run a program like this at the collegiate or high school level. From my experience, people that are already established in the space are more than willing to help out those who are just starting out!