Puellas, is plural accusative. I expected rather plural dative "to the girls".
Could someone explain a bit?
Edit: I get it now. Thanks to the forum, and the very helpful helping users here.
It's a double accusative here. Docere takes the accusative for the object, what is taught, and also, and it's unusual for us, for the person to whom it teaches.
There's an explicit preposition 'ad' required to be usef with the accusative for the 'to' to be present. Because the 'ad' preposition isn't in this sentence, it would be improper to translate 'to'.
For the sentence to read 'You teach to many girls', or more literally, 'To many girls you teach', the latin would be 'Ad multas puellas docetis'. Here you can see that the 'ad' preposition works with the accusative 'puellas', which is modified by 'multas', (adjectives agree in case, number, and gender, which is why it has the same ending) to introduce the word 'to'.