What a terrible position to be in! That seems really unfair to you as well as the other teachers who are going and the one who wont be because of her son. Do you feel comfortable talking to her about it? To be honest I wouldn't be because I hate confrontation but if you are and don't think it would cause any problems long term then maybe you should. Could you talk to the person who is in charge of the whole thing? It sounds like something administration should deal with but I don't really understand the way things are set up. Sorry, I was zero help right?
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"If you have men who will exclude any of God’s creatures from the shelter of compassion and pity, you will have men who will deal likewise with their fellow men." — St. Francis of Assisi, 1181-1226