In the Teacher Dashboard, under the My Classes tab, select the class you wish to assign a chapter to. In the student roster, checkmark the box next to a student's name (or checkmark the box next to All Students to select all students in the class), select the Assign Content button from the Manage Class menu, then select a chapter from the dropdown and click Assign.
Once a new chapter has been assigned, it may be necessary to unlock modules for your students to continue. Locked modules and levels can be identified by the 
icon. In the Select Chapter dropdown menu (found next to the Curriculum Guide button), select the newly assigned chapter, checkmark the box next to a student's name (or checkmark the box next to All Students), then hover your mouse over a module's padlock icon 
and select Unlock to unlock all the levels from that module. Modules and levels can be unlocked, skipped, made optional, and even locked indefinitely or until a set date.
Clicking the padlock icon in the top right corner next to Revoke Licenses will select all modules and students automatically and allow you to update them all at once.