This a great challenge to give your chemistry students. I prepare a flask in advance so the students can examine an example to understand the goal. Before trying each attempt I ask students to record a description of what they are trying and why they think it will work. After each attempt, students record their observations and if necessary a new technique based on their previous observations.
Students are to think about how temperature and pressure changes may have caused the balloon to be sucked into the flask. Be sure to discuss safety rules regarding using the hot plate, and especially make sure if students choose to heat the glass flask to make sure that there is water in the flask so the flask does not break.