* Age: Primary school children have far fewer classes per day than secondary school or university students.
* Type of school: The curriculum and class structure differ between primary, secondary (grammar, comprehensive, independent), and further/higher education institutions.
* Subject choices: Secondary school and university students choose subjects, leading to a varying number of classes per week. A student taking three A-levels will have fewer classes than one taking four or five. University courses also vary widely in class load.
* Year of study: University workloads often increase throughout a degree.
In short, it's impossible to give a definitive number. You'd need to specify the level of education (primary, secondary, university), the type of institution, and even the specific course of study to get a reasonable estimate.