And two easy to implement solutions:

1. Display it on the screen, tell them to keep in mind that there might be a confusion.
And if the first attempt fails they might try with an "O" instead a "0" or viceversa.
You may adapt the twig-file to show this message.

We are carrying out a survey with high school students.

They shouldn't be too stupid not to understand this.

In one of my programs there is the phrase:
Es gibt kein kleines "l" wie "leider falsch"; es ist immer ein großes "I" wie "Ich mache es jetzt richtig".

2. We do not know how these respondents receive the token - probably by mail or it is displayed on the screen in your external script.
There certainly is a way to use a different font that distinguishs better
E.g.
Roboto Mono
 
Ubuntu Mono
 

Joffm

And, as @holch: Now this feature is available, in version 5.x.

We developed a tool for that: github.com/toolsforresearch/TFRcustomToken

It is now part of the default installation of LimeSurvey 5.

We cannot give free support on the plugin. We suggest you upgrade to version 5 of LimeSurvey.

@tammo,
That gives me more confidence in my memory.
I remembered there was a discussion about this and plugin but had no clue at all where.

Joffm
 

Dear all, thank you very much for all the suggestions. The outline on how much longer LS3 will be supported and that LS5 will soon be the new standard as well as the official implementation of this functionality in LS5 provides confidence, that an upgrade might be the best way to solve our problem. We hope we can test this functionality in the holiday season. Thank you as well for the suggestions with the font and the notification: we already chose a font that distinguishes at least "O" and "0" well, but often respondents do not have a comparison (only a single character of those two appears in the token) so the confusion persists. We added the suggestion to try "O" or "0" (and "I" "1", "l" respectively) if a wrong token was provided and hope to catch some frustration already with this step.