Most seven-year-olds can spell simple words such as "hat", "net" and "fish", says the School Curriculum and Assessment Authority.
But "friends" and "bucket" are beyond them, according to this week's analysis of pupils' performance in the national curriculum tests. Hardest of all was "picture".
Most 11-year-olds could cope with "tallest" and "replaced". But only half could spell "shook" or "beautiful", and only a third managed "stretched" or "sneezed".
Overall, the results for seven and 11-year-olds in maths, English and science show a slight improvement on last year. According to SCAA, this is largely because children were better prepared for the tests.
But the authority remains concerned about some weaknesses, including: an over-reliance on simple phonic strategies for spelling, leading to problems with complex words; an overly literal view of writing; the over-reliance on fingers and buttons as aids to mental arithmetic; and a lack of progress at the primarysecondary school transfer age. At the same time, SCAA praised progress in basic number work, science and, at the upper age range, in essay writing.