Thank you all for your help !
Best solution is indeed to create a custom field, thank you Gavin. 
Here is the reason why I need to display email addresses for every members :
The Discourse forum on which I’m working is designed for my school network.
The goal is to create a community for graduates and non-graduates people.
Thus, displaying email addresses is a good way to share useful information.
Hawn → just because I’m too curious. How could I have modified the file user_guardian.rb in which the method can_check_emails is stored ?
I know it is possible to modify templates such as .hbs files with plugin-outlet.
But it seems the following script is only designed for template, and not for .rb files :
<script type="text/x-handlebars" data-template-name="/connectors/PLUGIN-OUTLET-NAME/UNIQUE-NAME">
</script>
Many thanks,
Richard