The Question and Answer section for Matilda is a great resource to ask questions, find answers, and discuss the novel.
I think Matilda needed to protect herself more than telling the truth. People would not understand her powers: she would suffer even more if they knew the truth. Matilda had a lot of antagonists in her life and she needed to protect herself.
The children are shocked and afraid of Miss Trunchbull.
Nigel Hicks garners Miss Trunchbull's attention while she is inspecting hand cleanliness. Nigel's are filthy.