Learn about structures and the stresses they undergo on a launch, the chemistry of rocket fuels and motors, the areas of physics called aerodynamics and fluid mechanics, electronics if you want Arduino or Raspberry pi based sensors in the rocket, and so on. This could be a nice way to build interest among larger groups of students and just start with purchased kits; learn from the kits, and let students begin to make their own designs and test them. Just something to consider if there is an interest.
At the college level, many engineering colleges and universities will have rocketry clubs. Check out USC's rocket, the first to have an all-student group build, from scratch, a rocket that surpassed the Karman line, which is the established boundary something needs to cross to be considered in space.