set priority for yum repository

Hello,

We want "yum install" to check the local repository before checking
other mirror repositories. Is this possible?

The man page of yum.conf says "If you list more than one baseurl=
statement in a repository you will  find  yum  will  ignore  the
earlier ones and probably act
bizarrely. Don't do this, you've been warned."

We would like to create a local repository, which only contains
essential packages. Even without Internet connection, students can
practice using yum to install certain packages. However, if the
Internet connection is available and the packages are not available
locally, yum will search the online repository (e.g.
mirrors.kernel.org). Is this possible?

Thanks,
Phil