The following lines are from the footnotes of the Mahabharata
In India, from the remotest times, preceptors are excluded from charging their pupils any fees for the instruction they give. No doubt, a final fee, called Gurudakshina, is demandable, but that is demandable after the pupil has completed his studies. To sell knowledge for money is a great sin. To this day in all the indigenous tols of the country, instruction is imparted free of all charges. In addition to this, the pupils are fed by their preceptors. The latter, in their turn, are supported by the charity of the whole country.
Although above lines say that selling education for money is a sin. It is not an explicit statement from scripture.
Is there any explicit statement from scriptures saying about correctness of selling education for money in Kali yuga?