Why is הן כולם correct? Shouldn't the gender agree?
הם כולם :should be the correct answer
Ok, good to know it's a bug.
The incubator tells me only הם כולם and הן כולן are accepted.
But in multi-choise mode it still offers הן כולם as correct answer.
It's still showing הן כולם in multiple choice.
This just happened, it still thinks הן כולם is a valid answer. http://imgur.com/XLkAXkk
The same problem still exists in multiple choice. Sounds like a problem with the system but just so you are aware.
Is כולן הן OK instead of הן כולן?
Well, כֻּלָּן הֵן all of them (= everybody) is a different construction in English and Hebrew, but the difference in meaning is very slight.
Doesn't it also mean "they all drive" or "they are all driving"?
No, drive/driving would be נוהגות. Note the additional vav.