Master painters who taught students did not hold classes or follow a formal curriculum. In general, painters in the 16th century were considered craftsmen, more skilled than manual laborers but no better than clockmakers, goldsmiths, furniture makers etc... Their training and their teaching was all done in their workshops. Apprentice painters started off doing basic tasks, like grinding pigments and making brushes. They would observe their teacher and more advanced students and learn by doing. There weren't formal classes, but a student would learn how to use perspective, layer colors, create shading, etc... on the job. If a student was good, he (or VERY rarely she) would take on more responsibilities helping the teacher fulfill commissions. Eventually, they'd breakout and start their own studios. To get accepted as a student, you'd have to demonstrate that you had some talent, usually by producing a competent drawing. It also helped if you had family connections in the business, which is why there were so many Carraccis, Bruegels, etc...