When my first child was preschool age we did not have formal school lessons. However, many of her friends were going to preschool so she wanted "school" also. So I acquiesced. I would say, "ok, it's art time," and would set the easel up outside to paint. Or, maybe it was "letter time!" and we would trace a letter and read an ABC book.
For Kindergarten, we would have "school time." This was still pretty informal, but it distinguished how we spent our time - Mom decided the activities rather than the child. Play time was when the child chose the activities. I think it helped to say "school" time, or "lesson" time if you prefer, because it sets the precedent that there are specific things to do. Does that make sense?
Now that my children are in 2nd grade and kindergarten, we still have school time, but I designate "Morning Class" (math, reading, writing, history, geography, Bible, etc.) and "Afternoon Class" (literature read alouds, art, nature walks). This helps me avoid power struggles with an independent child who would gladly do her own thing, but will do what Mom has planned because it is expected of her to her school work (the "I Will " part of CM's motto).