I was thrown into the deep end.
When I was 20 years old, I became a substitute teacher. I was still in college, but I was also looking to make some extra money when I was home for Thanksgiving, Christmas and the subsequent spring and summer breaks. The pay was OK, but the schedule was very flexible. The administrator would call me up the night before and ask if I was free to take over whatever was in need of adult supervision: social studies, science, physical education, English, etc.