So I'm not available on Tuesdays or Thursdays because I am in class for 6 hours and I still want time for homework and self care. Lately my boss has been telling me we are short staffed and the other day tried to ask me to come in on a day I wasn't available. I told her no, but I'm worried I will be asked to come in on a day I am not available and I will be asked why. I don't really want to delve into why because I've had previous bosses try to argue with me and still try to get me to work before. I know I am not obligated to say why I am not available, how can I politely refuse to tell my boss why I'm not available on a certain day if she asks?