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You can run simulations perhaps, but it's not really possible to test this outside of production.

Running the routes on the real streets on a non-school day would show if your routes are completely non-feasible, but traffic patterns are different on school days. The first week of school is almost always worse than the rest of the weeks because there tends to be more bunching.

With a totally new transit plan, there's going to be a lot of bottlenecks that are hard to discover.

You could maybe run a test of the new routes during the school day toward the end of the previous school year, but you would need an extra fleet of busses and drivers, so that's a big expense. Not to mention, some school districts finalize their transit plans weeks before the start date when enrollment is firm.