You can use hide/show to hide some part of your diagram (see http://forum.plantuml.net/7337/classes-then-show-only-classes-bearing-given-class-diagram )
For example, you can have :
@startuml
class Car
Driver - Car : drives >
Car *- Wheel : have 4 >
Car -- Person : < owns
hide *
show Car
show Person
@enduml
http://www.plantuml.com/plantuml/uml/BOqx2WCn34JxESNK2-vIXH0MaWEaIorYWHOS6wJzN3zss4xlQPYPOnLVjs_1fqW47-B0qvTTdMaGhym3WtSCd1BVfbgwDzcLbzEdn9TwjDeVDxQZ1c1BLau8QyU_U1nd2fYrvhxx0m00
If you change hide and show statement, you show part of the diagram.
It's even possible to use tag to group class together:
@startuml
class Car $material
class Wheel $material
class Driver $human
class Person $human
Driver - Car : drives >
Car *- Wheel : have 4 >
Car -- Person : < owns
hide *
show $human
@enduml
http://www.plantuml.com/plantuml/uml/PSqz2W8n4CNnVaxn2gk5T5P1P44FO6Szc844yW6PPFVwxafIMSwFntzMRLnRJv4-aLNnu8fBuYOryBIt5ub_-gnXasFiU-8yx2LLInu6eh4nlwo5Eq_5dSvpCIDiuNaJN8SRCpCMDvGzAv4FJh2G-hBFXwjarrFy0W00
It this what you are looking for ?
And of course you should use preprocessing option (!include) to avoid duplication for your slides.