Hi LISAL,

There is no feature in Khan Academy that allows you to hide assignments once they have been assigned. One thing you could do to avoid assigning all the content at once is to schedule it. 

There is now a feature that allows you to schedule assignments for a later date and these won't show up on your students' accounts until the date you set in them arrives. To know how to do this, be sure to take a look at the article How do I schedule an assignment for a future date?

Hope this helps!