There is no such setting.
Properly, it should be balanced on the last page, IMO.

They may be selecting the text, not including the last paragraph mark, when they set the columns. This will give a continuous section break at the end and produce the effect you are seeing.

00 deleteme 1.png

Showing non-printing formatting marks in Microsoft Word

If they are selecting paragraphs they will get this result.

Columns

Bottom line, the teachers should be teaching students to do their work with the paragraph marks displayed.

Also, if they simply click in the document and set two columns, they will not get the section break.

You may want to have someone look at a good style manual on the unbalanced column issue, though.

If your students work in documents with multiple sections (most long documents) the columns will end up balanced in most cases.