The Scottish poverty and inequality research unit at Glasgow Caledonian University has been looking at the school meal service.
The team found about a quarter of the primary children registered for free meals do not have one on a typical day.
The proportion is even higher among secondary school pupils. Overall, 350,000 school meals are served daily.
Close to 100,000 free school meals are also served every day to pupils who are entitled to one because their families are eligible for social security.
The research was commissioned by the Association of Facilities Managers (Assist FM), whose members include every Scottish local authority school meals provider.
Jayne Jones, national chairwoman of Assist FM, said: "This report raises some very important issues for school meals services.
"All of our members are dedicated, as local authority providers, to ensuring that children receive a healthy nutritious meal at lunchtime. For many school pupils, the only hot meal they will have each day is at school.
"However, there are areas that we need to explore further, such as what happens outside the school gate, as well as the support we need internally within schools to achieve better uptakes, especially for pupils who are entitled to free school meal provision."
All pupils in P1, 2 and 3 in Scotland are entitled to a free school meal - after that, they depend on family circumstances.
However, some councils exceed their statutory obligations. For instance, all P4 pupils in Glasgow are entitled to a free school meal and the council plans to extend entitlement to all primary school pupils before the next local elections.
Uptake across Scotland was higher in primary schools and in schools which were either smaller in size or more rural in character.