"The cats wrote a lot at school yesterday."
Translation:Los gatos escribieron mucho en la escuela ayer.
Why can they not be female cats. I wrote Las gatas escribieron mucha en la escuela ayer This was wrong. They had to be male cats
"The cats wrote a lot at school yesterday." there is nothing in this sentence to tell you if the cats are mixed, all male or all female. The same applies to the spanish sentence hence the use of Los gatos.