Canker sore at the back of my daughters throat?

My daughter, who is 11, mentioned this morning before school that her throat was hurting quite badly. I took her to school, and came home. I went to pick her up and about 20 minutes ago, she said "Mom. Look at this." Well, she wanted to look at her expander that she got yesterday afternoon a little closer. So she put the flash on her iphone on and she took a video of her mouth just to see everything, being the curious little girl she is! Then, she showed me the video. She had definitely noticed that there was a white/yellowish patch on the right side of her throat. I had noticed it immediately. I said "Hun is that where your throat hurts?" And she said "Yeah. That's why I came to show you. What is that?!" It was a canker sore. I told her to take an Advil and go lay down. She's never had strep throat and the pain is just above her right tonsil. The problem is, she started school 4 days ago. I can't take her out the 1st full week of school! If I have to, I definitely will, but she loves going to school! The orthodontists were having trouble fitting her expander yesterday. Does that have anything to do with it? But I don't want her gargling or swishing anything in her mouth, because its hard for her to keep liquids from going down her throat right now. All help is appreciated! Thanks so much! Oh, one more thing. Are there any remedies (besides liquids) that can heal it within the weekend? Should I take her to the doctor?

My 3 1/2year old son has the same thing. This is his second go-round with this. The first time he was in so much pain he couldn't eat and barely drank. His pediatrician said there isn't much to do other then Tylenol and sore throat spray for the discomfort, warm soup and milk will help soothe but its mostly just a waiting game. His last one cleared up in about 6 days.