Choose a given topic you intend to teach for the week.  In each class, choose two open questions intended for the students. Since secondary school teachers teach multiple classes of the same grade, revise (not changing completely) the question for each class. In one month, check the number of open questions and see how you can improve on it for the next topic.   A second approach is to get students to summarise what they have learnt. In the summary, they should write out the mathematical steps they follow in solving the questions and speak it out verbally.