Is there a law to say that students may leave the class if teacher show up at start of lesson and still shown for 20 minutes?

There is generally no such law. In most cases, students are required to remain in class for the entire duration of the scheduled class period, regardless of whether the teacher is present or not.

However, some schools may have specific policies or regulations that address situations where a teacher is absent or late. For example, some schools may allow students to leave class after a certain period of time if the teacher has not shown up.

If you are concerned about your school's policy regarding student attendance, you should consult with your school administration for specific details.