I imagine that students using products like Blackboard and TurnItIn in middle-schools and high-schools would want to use a PC.
Blackboard is a course management system used in high-schools and colleges to manage announcements, assignments, online collaboration, syllabi, scheduling, grades, etc. It can be used in online and in-person classes.
TurnItIn is an online tool to check your papers for plagiarism and many professors submit student papers to verify that there aren't plagiarism issues. Students can submit their papers before emailing them in.
Many textbooks today include an online component which is often easier to use on a PC than on a tablet. Sometimes you can't do them on a tablet when PC software components like Flash are required. The reason for doing this is to make less useful the ability to resell textbooks. It also reduces the amount of work that teachers have to do in coming up with tests, grading them, etc.