Spaceref has some posts about student challenges to design a plant growth chamber for astronauts on the Moon and to present their ideas on the aircraft of the future.
The aeronautics contest is divided into high school and college levels. The site doesn't seem to commit to specific cash prizes (they depend on availability of funds), but
Cash prize amounts will be on the order of $5000 for first place, $3500 for second place, $2000 for third place. In the case of a tie, prizes will be determined based on funds available. Non US schools are not eligible to receive cash awards.
Internships are also possible. The suggested first team and individual prizes for the high school level are $1500 and $1000.
Aeronautics entries that address space goals are possible, and have won awards in the past. For example, this year the Mars Heavy-Payload Entry, Descent, and Landing Team from the Georgia Institute of Technology tied for third place.