They'd be graded as a mistake. But it is an interesting question also from a linguistic and historic point of view. Because septante, huitante/octante nd nonante used to be the proper way in France, too, up to about 100 or so years ago.

Here is a discussion you might find interesting: https://forum.wordreference.com/threads/french-numbers-septante-huitante-octante-nonante-soixante-dix-quatre-vingts-quatre-vingt-dix.4205/

And for the record, I find quatre-vingt-dix-neuf a ludicrous way to cobble together 99: 4-20-10-9? Come on! :)

In Belgium they use Septante, Nonante too :)

I've never heard anything but quatre-vingts in Belgium. Nonante is pretty much universal. I think septante is generally used too (but it turns up less often in prices).