You can assign TeacherMade assignments to multiple classes in your Google Classroom at one time! Start by creating your assignment as usual. On Step 2, you will see this:

These boxes work like checkboxes so you can select as many as you would like! Each assignment is automatically set to assign to all students. If you want to choose which students in a class receive the assignment, click ‘Select Students’ and select the individual students you want.

Continue creating the assignment like usual. Your students’ submissions will still be separated by classes in your gradebook!