New tougher SATs "ensure" schools help children master basic employment skills such as literacy and numeracy.
Campaign for Real Education chairman Chris McGovern criticised Tuesday's "Let Kids Be Kids" campaign, saying using children as part of the protest is a "sure way of upsetting their sanity and wellbeing".
Mr McGovern said British school-leavers are shown to be doing "very badly" and believes the UK has become an "education basket case".
OECD figures reveal by the age of 15 UK youngsters are up to three years behind 15 year-olds in parts of the Asia-Pacific.
Mr McGovern said the latest figures are tantamount to a "betrayal" of the younger generation.
"Yes, we need to let kids be kids but we need to ensure that they have a future, too", he says.
"SATs are an educational health check. For short-term pain they provide long-term gain".
Answer these sample questions to find out how you measure up against primary school pupils aged up to 11 years old.
Parents will be taking their children out of school tomorrow in an unprecedented "strike" against compulsory exams for six-year-olds.