Best Children's/YA Science Fiction

Question to the collective wisdom here. . .
We may be getting some money and we are looking to add to our Children's and YA Science Fiction (*not* fantasy) collection.

Which titles would you wise and wonderful people recommend for
Grades K-5 / 6-8 / 9-12?

Title/author with grade levels would be most helpful.



There is a site here called the Golden Duck awards for Children's SciFi. Maybe it could be of some help?