Quartus II 14.0 schematic editor, signal names not visible

I am teacher of digital courses, we are using Quartus II, version 14.00. Sometimes in the class students complain that when using the schematic editor, the names of the signals (pins and labels) are not visible. They indeed seem to be covered by something. Trying to change the font size does not do it (the font size remains the same). It can be frustrating for the students since they do not see what they do.

I do not have this problem on my computer, but I've seen it on others. Today I saw it on a Windows 8.1. Is it a bug or a wrong setting? How can it be solved?