It’s very, very unusual to write marks in words. I have only ever received a mark as just that: 8.5.
8.50 (eight point fifty) sounds to an English speaker as if you’re talking about £8.50 in money.
If you have to write it in words, eight point five (8.5) is the way to go. But as I say, I’ve never seen marks written out as words, only as numbers.
Hope this helps!