You can assign lessons from many pages! Including:

Step 1:  Go to the "Library" page
Step 2: Click the arrow.

Step 3. Click which lessons you'd like to assign to which classes. You can choose to assign all lessons or specific ones. Press "Done"
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Step 4: You're done! You can choose to share this unit to Google Classrooms if you'd like, but you don't need to. When your students log into their accounts, they'll automatically see the lessons you've assigned.

Click the "More" button then the yellow "Assign" button. Then, follow steps 3-4 above.

Step 1: Go to the unit you'd like to assign.
Step 2: On each page, you'll see a green "Assign" button.
Step 3: Click this button and follow the instructions above for step 3-4.

Step 1: Click "Assign Units"
Step 2: Select the unit you'd like to assign.
Step 3: Click the save button.