Kan jag säga 'måste gå'
Yes, absolutely. (not on the listen-and-type excercise tho ;) ).
I think the sentence is unnatural or has an error, because there is no main verb in it, I've reported it but, please if I'm wrong can anyone tell me the reason or explain the sentence to me!
There's nothing wrong with it, actually. Måste can be used like that to express "have to go".
Det är sent, jag måste hem = It's late, I have to go home.
Like the English 'I must away.'
Is that common?
Like in the Hobbit: We must away ere break of day.
It's archaic, but the same structure.
I see. Didn't know that. :)
Haha! I still use that and I'm only in my 50s :) Actually quite a lot of people in Scotland say that. I must away now
It wouldn't be wrong though to say 'Ni måste gå till skolan nu'.
This is similar to the constructions vill till and ska till meaning want to go to and shall go to (or will go to) respectively. I wrote something about the construction vill till here: https://www.duolingo.com/comment/5884363
German has the same grammatical feature.
Ich muss jetzt in die Schule - I have to go to school now
Ich muss jetzt ins Bett - I have to go to bed now
I guess it's because it's kind of obvious, so the verb 'to go' isn't necessary.