Going back to high school at 18?

I am graduating this year (Australia) and I know my score won’t be enough to get into medicine at uni. I’m thinking of going back to High school and getting the score I am positive I can get.

I was diagnosed with chronic migraine disorder last year and destroyed my grades while I was finding the right medications. Now I get migraines 1-2 times a week but they’re tolerable.

Do you think it’s ok if I go back to high school (I’ve contacted two schools that are willing to take me) and one of them has a rank of 70 while the other is 170-180.

I just want a second chance and I haven’t been keen on tafe. If I do cae then I cant study anything for a year (what I was told), so I figured moving to Queensland and going to a mature age high school would be best.

I just finished hsc. Wanted to do vce but have to wait, therefore qce seems ok.

Is it true that I’m wasting my time? Do you think it’s a stupid idea?

Medicine and university is 100% my ultimate goal. I’ve never thought of anything more important.