This is something you should bring up with the teacher. I wouldn't supply details, but simply ask about "the incident with the tray" yesterday. Find out the teacher's version of the story before you assume your child got all the details right. When upset, we frequently hear things that weren't actually said, or miss things that were. And children do have great imaginations.
Did your son come home with food on his clothes? Could that have happened as he dropped the tray? Could there have been a food fight between kids that ended in messed up clothing and a need for a story? Other possible explanations? Try to hold an open mind before blaming the teacher for a rather absurd request. Teachers do get accused of all sorts of weird stuff that never happened, because they are not there when the student is trying to explain things to Mom or Dad.
I'm not suggesting your son is lying or mistaken, necessarily. He definitely needs you to back him up if something inappropriate or unprofessional did happen. The simple fact is that kids' interpretations of what happened is often lacking perspective. They can't help it – their perspective is quite limited.
The teacher may be able to reassure you if you'll hear her out. But if she seems evasive, she could be embarrassed or guilty. At that point, you might offer some of the details your son told you, and ask what happened that would have given him that idea. A hasty altering of her story, or dismissal of your son's distress as foolish, or similar defensive or uncaring behavior would leave me feeling this is not the teacher for my child.
As far as comforting a child who's upset, often the best thing to do is to hold them and make a simple, direct observation, like "Oh, sweetheart, I see that you are really sad/angry/scared (or whatever feelings you notice) about this. Here, let me give you a hug. Do you want to talk more about it?" Then just listen. Don't lead the child to supply more detail than he offers without coaxing, or he may feel the need to "embroider" the actual event.