The first option is improved by changing it to:
"If you want to leave a comment that all exam participants will (be able to) see, click here"
The second option sounds the point of the comment is for everyone to see it. The first one sounds more like a note or warning telling people that the comment is visible. But may not be for the sole puprose of being shown to everyone.
The addition of "be able to" changes the meaning slightly from meaning that the comment is likely to or definitely will be seen by everyone to meaning that the comment could be seen by people if they chose to look at/for it.
You can choose between the options based on the intended meaning. The difference is small however, so whichever one you choose will not sound wrong and will convey the general meaning well.