26 on ACT with no prep

I took an ACT diagnostic test with no prep and got a 26. I am a sophomore in High School in the United Staes. I am looking score a 34 on the ACT and am willing to prep an extreme amount. Is it realistic that I can score a 34 on the ACT?

You can certainly improve your score. My S raised his composite score a lot with private tutoring (about 8 sessions).
So, yes, it is possible but you should do some prep, take a mock test, evaluate the areas in which you need to improve and then take the ACT. Good luck!

Eventually, yes. And then they will accuse you of cheating:

http://talk.collegeconfidential.com/sat-act-tests-test-preparation/1987200-act-testing-wrongly-accusing-cheating-2017.html#latest

Yes it is possible. I am a high school senior and I took the ACT last year without any prep. I scored a 24. I then decided to pay for the $40 course that college board offers on their website. After roughly 20 hours of studying with that course, I took the ACT again and scored a 30. I then took the SAT after another 15 hours of study and scored a 1430, the equivalent to a 32.

If you study a little bit, you will see an increase in your score. If you study a lot, a 34 is possible.