Add more than 20 students to an assignment

Hi there

Rather than assigning an assignment to all students, I'm trying to add a group of 20+ students to it within a Team, but when I get to 20, it won't let me add any more.  Is this a "thing" or is there a way around it?   

(As an aside, at the moment, it's not even giving me the option to assign the assignment to anyone except "All Students"). 

Thanks in advance. 

sarah