OP, congrats on your trip to Universal
We love Universal. It is a more relaxing type of touring than WDW.
We went for one day the first time with no hopper, to Studios side. We had a ball but felt we missed out on alot of things due to just one side and one day.
We have since gone for 2 days, park hopper, but would like to do more.
As for the teens only wanting to do thrill rides? Ours loved it all, even had a blast at the Make up Horror Show. Not to be missed IMO.
Also we are retired and Love Universal and all their rides. So it is not just for teens, LOL.
I don't know your situation, but would definitely do 2 days at Universal (or more) especially if you are doing Volcano Bay. You could keep the one day for HS too. But I wonder if you will be able to get any ILL's at all? I have read on here that most are gone by onsite guests within minutes? It's something to think about if you are doing stand by lines only.
I know you will love the vibe at Universal - being able to walk around and ride what you want when you want - just like disney used to be. Have fun!