Regarding AROTC. I'd say it's a bit harder for in-college vs high school
The criteria of which you are assessed on isn't too different. It's based on your grades, PT score, extra-curricular activities - alot like that in the high school scholarship program.
The big difference lies within the environment with which you study in. In high school, pretty much everything is stream lined for you. Classes are scheduled, you probably have an after school sport to play, and homework probably comes shortly afterward.
In college, things are a lot different. For starters - you now have new-found freedom. There's no one there to tell you to go to class, do homework, play a sport, etc. If you really wanted to, you could literally just putz around ... at the expense of your grades of course.
My point is, in college, there are a lot more distractions present. Parties/socials, clubs, sports, and a whole lot of other things you'll find out.
And because of this, it can be a little harder to keep your grades up.
Regarding "said cadet + whole graduating class on scholarship," take that with a grain of salt. For all you know it could be some small school with a class of 40 cadets, with 10 kids per graduating class, with that program being allotted 10 scholarships per year. So yeah, easy for them.
Then you compare it to a Big 10 school. 300+ cadets, probably 40-60 cadets per class. You get 30+ students competing for 10 scholarships ... not so easy now is it?
OP, at this point there's nothing you can do to control your fate, as to whether you will get a high school scholarship. Stop worrying about it. Enjoy the rest of high school, and finish it out strong. Control what you can, not what you can't.