The Oxford University Press says the problems occur because pupils cannot spot when words such as 'their' and 'there' or 'cloths' and 'clothes' are misspelled.
The Telegraph reports that the researchers analysed pupils' spelling skills using the Oxford Children's Corpus - a database containing the written work of almost 75,000 children.
It found that most commonly, students failed to pick out silent letters, or the difference between a single or double letter in words likes 'disappeared' or 'tomorrow'.
The word they most often misspelled was accidentally, followed by practising, frantically, definitely and believe.
Other words which tripped pupils up were surprise, excitement, weird, doesn't and minute.
Vineeta Gupta, head of children's dictionaries at the Oxford University Press, said that the findings would enable more targeted help for pupils.
"Spellcheckers can be useful but may not provide all the support a child needs to distinguish confusables such as their/there and cloths/clothes," she said. "These findings are fascinating and give us an opportunity to target the areas children need more support in."