Are Scantron Sheets under Copyright? Or are there non-proprietary alternatives?
The scantron forms and similar forms are not subect to copyright. US Copyright Office Title 37: Patents, Trademarks, and Copyrights §202.1 Material not subject to copyright. (c) Blank forms, such as time cards, graph paper, account books, diaries, bank checks, scorecards, address books, report forms, order forms and the like, which are designed for recording information and do not in themselves convey information. 

So to answer your question, no. As long as the school or university doesn't photocopy the actual scantron sheets from Scantron. inc. But it's not illegal for them to create their own version of the scantron sheets that can be readable by the official Scantron machine itself. 

Plus, there are numerous companies, not even associated with Scantron. inc. in any sort, That sell scantron compatible sheets for cheaper prices than the 'official' scantron sheet. Amazon and eBay are full of them. So, yeah...