President Toure has been urged to free those detained
The five journalists and the teacher who set the essay to his class are due to appear in court next Tuesday.
Teacher Bassirou Kassim Minta asked his final-year secondary school class to write a humorous essay about the mistress of a fictional African leader.
The arrests have been condemned by Reporters Without Borders.
The US-based Committee to Protect Journalists (CPJ) called the arrests "outrageous".
Journalist Seydina Oumar Diarra wrote an article called The Mistress of the President of the Republic, in the Info-Matin newspaper about the essay.
Police then arrested him and Mr Minta.
Reporters Without Borders has urged President Amadou Toumani Toure to release those detained.
"The result of a prosecutor's absurd zeal, these two arrests are worthy of another age and are clearly an abuse of authority," the press freedom group said.
"Mali was hailed as an example of democracy in Africa, but as this case goes from bad to worse, it is looking more and more like an authoritarian regime, crippled by taboos and dangerous for those who show a lack of respect for an untouchable president."
Earlier this month, the World Association of Newspapers (WAN) urged African countries to scrap their laws on insulting leaders at a congress in South Africa.
Its declaration said such laws, which are in force in 48 out of 53 African countries, were "the greatest scourge" of press freedom on the continent.
President Amadou Toumani Toure was last month re-elected for a second five-year term in first-round presidential elections.
International monitors said the vote appeared to have gone smoothly, but opposition candidates alleged fraud.Mr Toure, known as "ATT", was hailed after ending Mali's military dictatorship with a coup 16 years ago and then stepping down after organising elections.