Translation?

[Nicolev]

How would I say "If I did highschool over again, I would have studied more."
Would I use the subjunctive?

[Rusty]

Give this a try. Yes, the subjunctive is needed.
The subjunctive mood goes in the if clause. The conditional mood goes in the main clause.

It might be easier to reword the sentence:
If I had to go to high school again, I would have studied more.
-or-
If I were to repeat high school, I would have studied more.

(Notice that high school is two words.)