The British Model Flying Association (BMFA) has a range of kits of gliders and rubber powered planes for youngsters to build and fly.  They are available  singularly or in bulk packs for groups.
The Teachers’ Pack includes some samples and curriculum linking work, and is excellent value. Their education section website has details and prices at http://www.bmfa.org/Info/Education/EducationProgramme/tabid/124/Default.aspx

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