I want to force following scenario in Jenkins:
I think if you run the System Groovy Script, Jenkins objects are available by default. What you are running is the groovy script. Hence, need to import the packages/classes.
For future readers, Tim's comment above is the solution:
import jenkins.model.Jenkins
I ran into the same problem "unable to resolve class jenkins.model.Jenkins" as commented by @iloveretards, until I realized I had tried to use the build step "Execute Groovy script". After switching to build step "Execute system Groovy script", this actually worked as intended:
import jenkins.model.Jenkins
Jenkins.instance.getItem("job-name").disable()