A family of Microsoft word processing software products for creating web, email, and print documents.
Regardless of purchasing a subscription to Office 365, Word Online does not have any of the Table functions such as Merge or Split. The document's table would have to be altered in the desktop version.
Since you are an educator, and have the free version, you should be able to load it on your desktop. But again, that does not change the functionality of the online version.