Schools have been warned that companies are unlikely to provide "something for nothing" when they pump money into education.
A Scottish Consumer Council guide for schools on commercial activities reminds teachers that companies are keen to market their products to children and through them to parents.
It advises they consider any disadvantages before taking advertising or sponsorship cash. Any gift of more than pound;3,000 should be referred to the local authority.
Commercial activity is now wide-ranging, with advertising on school minibuses, sponsorship of materials such as packs on money management and voucher schemes for books and computers. Companies also sponsor events and run prize or reward schemes.
But supermarket voucher schemes can tie up staff time, while food or drink products should be checked for their nutritional value and whether they meet the standands of the Hungry for Success national campaign.