According to experts behind the government-funded report, parents' anxiety may be rubbing-off on the kids.
In the study, researchers from the universities of Bath and Bristol analysed levels of the stress hormone cortisol in the saliva of 105 four-year-olds.
At the beginning of a term, average cortisol levels were 5.47 nanograms per millilitre of saliva, which is high for a likely reaction to a new situation.
Six months later, after the children began adjusting with new surroundings, the level had fallen to normal levels of 1.42ng/ml.
But, during three to six months of the session, the levels were at 4.48ng/ml, which is three times more than normal levels.
"This suggests that stress levels in anticipation of starting school begin to rise much earlier than we expected," the Daily Mail quoted Bath University's Dr Julie Turner-Cobb, who led the study, as saying.
Dr Turner-Cobb said that "stress was being passed on to the children" by worried parents.
She added that it might lead to 'more long-term stress response that could lead to poorer health'.
Outgoing children are more likely to have higher levels of the hormone after the beginning of school term.
"Possibly because their more impulsive nature gets them into more confrontational situations", said Dr Turner-Cobb.
The study also revealed that these kids were less likely to pick up colds in six months and took fewer days off.
Dr.Turner-Cobb believes that high-stress levels may be unfavourable in the beginning, but provide some short-term resistance form cold.