I just received an email from Lumin PDF announcing that all of the premium features are now available to teachers and their students for free! The premium features include merging and splitting PDFs, highlighting (in addition to the normal annotating options), and the removal of banner advertising. And Lumin PDF integrates with Google Classroom. You can register for a free Lumin PDF premium account here.
If you're interested in other options for annotating PDFs, I recommend trying the tools built into OneNote or trying Kami. Video tutorials on how to use both of those tools are available here.