If the general form of address for an adult woman is "Madame", how do you address (e.g. on a posted letter) a young girl? Does she have a title, or do you just use her names without any title?
For something like a magazine subscription, Mademoiselle (Mlle) is often still used when the recipient is a young girl.
Madame (Mme) is possibly gaining momentum but I have yet to see it used for material addressed to primary or secondary school female students1. It's still either mademoiselle or just the names.
Nowadays, unless the girl asks to be called madame or mademoiselle, the safest approach is to just use the names.
Madelle was attempted in the eighties but failed to gain adoption so was abandoned. See http://bdl.oqlf.gouv.qc.ca/bdl/gabarit_bdl.asp?id=3634
1 However, the high-school diploma (Baccalauréat) convocations started to use Madame instead of Mademoiselle around 2015, but the average age of students is very close to 18 years.