Rockets are an enthralling concept. Background science for all ages is at ESA Kids: Launchers.
A simple paper rocket can be constructed in seconds (my record is 20 seconds). But this rocket is fired using the same science as its much larger cousin that blasts into space. The paper rocket instructions can be found here.
For second level students ESA’s Rockets and Technology Resources are a great starting point.
And if rockets is too limiting – then check out the educational resources from the main World Space Week site.