Troubleshooting at the LAN level comes in about two grades. At the simplest level, this involves inspection of LEDs at various components, isolating and replacing failed equipment. There is a theory of LAN operation that fully qualifies as education rather than training. Still well within the reach of high school students. The notion of internetworking multiple LANs and WANs involves abstract thinking that is a significant step above the structured wiring and LAN topics.
Theory includes routers, routing information protocols, and how they work.
Today these topics are typically taught at the upper division college level, but that is less a function of the students' ability to understand and more a function of where the capable faculty are. Once the network plumbing is in place, several services must exist in order that the network run. Bookkeeping and billing when in fee for service situations is just as important in networks as any other for-profit service industry. We should note that this work is people intensive and exists not just during installation and setup, but over extensive life cycles. People with these skillsets will find jobs in the industry for a long time to come.