Hi Shiran,
Are you able to post a screenshot or tell me what settings you have for your custom certificate plugin, specifically the email settings (if you have 'Email others' set you can blur the emails to keep them anonymous)? It may be broken where it sends the certificate too often to teachers but would be good to see your settings first before I investigate.
Also, please let me know what version of Moodle and what version of the plugin you are using.
Regards,
Mark