As a means of stimulating the improvement of teachers in service in the public schools of the state, institutes shall be held annually, one or more in each county or for a group of counties, at such times and at such places as the State Superintendent of Education, after advising with the county superintendent of education, shall direct. The duration of institutes shall not exceed four days in any one year for any county or group of counties. The character of instruction shall be such as to promote the best interest of the schools.