They were asked in explicit language to write about things that annoy them at Bishop King Church of England Primary School in Lincoln.
Pupils in a Year 2 class were given the work last week, asking them to write about 25 different topics, including their favourite city and number or season.
But task 22 on the list instructed children to give their views what they did not like.
Bishop King's headteacher Kate Rouse said in a statement: "It is with deep regret that a piece of homework was given to the children of one of our classes last Thursday in error.
"This was not intentional and letters have been sent to parents to explain and apologise.
"It wasn't appropriate to give this document to the children and I apologise for any distress caused.
"I'm grateful that many parents have been understanding and supportive but of course I can understand some will have been upset."