Do you have an email address you can contact them through? If so, then that might be an easier option for you.
I know this is probably not what you want to hear, but you should try and pluck up the courage to ask them about it. At the end of the day, they're there to help you get the best grades possible, and aren't going to be annoyed if you ask for help.
Going out of your comfort zone and approaching a teacher - perhaps after a lesson with them - will do you loads of favours in the long run.