Would online high school give me more time for dance training?

If all you do is 8 hours a week then you are not in a professional level school for a ballet career even if you do way more hours a week there. That small amount would not even be an option in a professional school for anyone over age 10

If you have what it takes for a ballet career, you will get into a proper professional ballet school on scholarship and so money should not be an issue. My daughter never paid for ballet classes after age 11.

You need to train at a world class ballet school if you want a chance of ballet career. Those schools are almost exclusively affiliate feeder schools to professional ballet companies. They will screen you for body, facility and musicality along with being at the level of training expected for a dancer your age or they wont train you. They will either assist you with your academics as often there are relationships with nearby schools or you could take online schooling. Based on where you are training now, it makes no sense to do online schooling as you are not in a professional school. You also cannot train at home as you need a sprung or floating floor to work on so you don't get injured, not to mention a teacher's eyes and hands on for corrections. Serious pre-professional ballet dancers only stretch at home and instead of home practice take multiple daily ballet technique classes. Even professional ballet dancers take a daily class with the company ballet master/mistress. If they can't do it on their own, how do you expect to do this successfully?

I suggest you look into auditioning for a professional ballet school for the fall term. Auditions are usually the day after labor day but check the websites for exact information. There are also a few schools like the Harid Conservatory which has free tuition for all who attend and a relationship with a local high school for their ballet students. While not a feeder school to a professional ballet company, this is one of the few exceptions.

* EDIT: Dancers on track for a professional ballet career train 20-30 hours per week year round at age 13 and over.

***EDIT: I just looked at your Q &A and see you are 16 which means you are very undertrained as you should be company ready at your age. On top of that you don't have good enough turnout according to one of your other questions for a ballet career. Dance for the Joy of Dance and dance can always be a part of your life. Don't give up your current high school for something that is just not a realistic goal for you.

Maybe, depends how long you spent doing the work because you would still need enough time